Introduce clear_user and clear_guest

Introduce clear_user for x86 and ia64, shamelessly taken from Linux.
The x86 version is the 32 bit clear_user implementation.  Introduce
clear_guest for x86 and ia64. The x86 implementation is based on
clear_user and a new clear_user_hvm function.  The ia64 implementation
is actually in xencomm and it is based on xencomm_copy_to_guest.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Jan Beulich <JBeulich@suse.com>
Committed-by: Keir Fraser <keir@xen.org>

12 files changed, 527 insertions, 0 deletions

diff --git a/xen/arch/ia64/linux/Makefile b/xen/arch/ia64/linux/Makefile
index 10ebbfc9e9..6a3704b1bf 100644
--- a/xen/arch/ia64/linux/Makefile
+++ b/xen/arch/ia64/linux/Makefile
@@ -4,6 +4,7 @@ subdir-y += sn
 
 obj-y += bitop.o
 obj-y += clear_page.o
+obj-y += clear_user.o
 obj-y += copy_page_mck.o
 obj-y += efi_stub.o
 obj-y += extable.o
diff --git a/xen/arch/ia64/linux/README.origin b/xen/arch/ia64/linux/README.origin
index 4dc96d3435..3727fe59d2 100644
--- a/xen/arch/ia64/linux/README.origin
+++ b/xen/arch/ia64/linux/README.origin
@@ -15,6 +15,7 @@ pcdp.h			-> linux/drivers/firmware/pcdp.h
 
 bitop.c			-> linux/arch/ia64/lib/bitop.c
 clear_page.S		-> linux/arch/ia64/lib/clear_page.S
+clear_user.S		-> linux/arch/ia64/lib/clear_user.S
 copy_page_mck.S		-> linux/arch/ia64/lib/copy_page_mck.S
 flush.S			-> linux/arch/ia64/lib/flush.S
 idiv32.S		-> linux/arch/ia64/lib/idiv32.S
diff --git a/xen/arch/ia64/linux/clear_user.S b/xen/arch/ia64/linux/clear_user.S
new file mode 100644
index 0000000000..eecd8577b2
--- /dev/null
+++ b/xen/arch/ia64/linux/clear_user.S
@@ -0,0 +1,209 @@
+/*
+ * This routine clears to zero a linear memory buffer in user space.
+ *
+ * Inputs:
+ *	in0:	address of buffer
+ *	in1:	length of buffer in bytes
+ * Outputs:
+ *	r8:	number of bytes that didn't get cleared due to a fault
+ *
+ * Copyright (C) 1998, 1999, 2001 Hewlett-Packard Co
+ *	Stephane Eranian <eranian@hpl.hp.com>
+ */
+
+#include <asm/asmmacro.h>
+
+//
+// arguments
+//
+#define buf		r32
+#define len		r33
+
+//
+// local registers
+//
+#define cnt		r16
+#define buf2		r17
+#define saved_lc	r18
+#define saved_pfs	r19
+#define tmp		r20
+#define len2		r21
+#define len3		r22
+
+//
+// Theory of operations:
+//	- we check whether or not the buffer is small, i.e., less than 17
+//	  in which case we do the byte by byte loop.
+//
+//	- Otherwise we go progressively from 1 byte store to 8byte store in
+//	  the head part, the body is a 16byte store loop and we finish we the
+//	  tail for the last 15 bytes.
+//	  The good point about this breakdown is that the long buffer handling
+//	  contains only 2 branches.
+//
+//	The reason for not using shifting & masking for both the head and the
+//	tail is to stay semantically correct. This routine is not supposed
+//	to write bytes outside of the buffer. While most of the time this would
+//	be ok, we can't tolerate a mistake. A classical example is the case
+//	of multithreaded code were to the extra bytes touched is actually owned
+//	by another thread which runs concurrently to ours. Another, less likely,
+//	example is with device drivers where reading an I/O mapped location may
+//	have side effects (same thing for writing).
+//
+
+GLOBAL_ENTRY(__do_clear_user)
+	.prologue
+	.save ar.pfs, saved_pfs
+	alloc	saved_pfs=ar.pfs,2,0,0,0
+	cmp.eq p6,p0=r0,len		// check for zero length
+	.save ar.lc, saved_lc
+	mov saved_lc=ar.lc		// preserve ar.lc (slow)
+	.body
+	;;				// avoid WAW on CFM
+	adds tmp=-1,len			// br.ctop is repeat/until
+	mov ret0=len			// return value is length at this point
+(p6)	br.ret.spnt.many rp
+	;;
+	cmp.lt p6,p0=16,len		// if len > 16 then long memset
+	mov ar.lc=tmp			// initialize lc for small count
+(p6)	br.cond.dptk .long_do_clear
+	;;				// WAR on ar.lc
+	//
+	// worst case 16 iterations, avg 8 iterations
+	//
+	// We could have played with the predicates to use the extra
+	// M slot for 2 stores/iteration but the cost the initialization
+	// the various counters compared to how long the loop is supposed
+	// to last on average does not make this solution viable.
+	//
+1:
+	EX( .Lexit1, st1 [buf]=r0,1 )
+	adds len=-1,len			// countdown length using len
+	br.cloop.dptk 1b
+	;;				// avoid RAW on ar.lc
+	//
+	// .Lexit4: comes from byte by byte loop
+	//	    len contains bytes left
+.Lexit1:
+	mov ret0=len			// faster than using ar.lc
+	mov ar.lc=saved_lc
+	br.ret.sptk.many rp		// end of short clear_user
+
+
+	//
+	// At this point we know we have more than 16 bytes to copy
+	// so we focus on alignment (no branches required)
+	//
+	// The use of len/len2 for countdown of the number of bytes left
+	// instead of ret0 is due to the fact that the exception code
+	// changes the values of r8.
+	//
+.long_do_clear:
+	tbit.nz p6,p0=buf,0		// odd alignment (for long_do_clear)
+	;;
+	EX( .Lexit3, (p6) st1 [buf]=r0,1 )	// 1-byte aligned
+(p6)	adds len=-1,len;;		// sync because buf is modified
+	tbit.nz p6,p0=buf,1
+	;;
+	EX( .Lexit3, (p6) st2 [buf]=r0,2 )	// 2-byte aligned
+(p6)	adds len=-2,len;;
+	tbit.nz p6,p0=buf,2
+	;;
+	EX( .Lexit3, (p6) st4 [buf]=r0,4 )	// 4-byte aligned
+(p6)	adds len=-4,len;;
+	tbit.nz p6,p0=buf,3
+	;;
+	EX( .Lexit3, (p6) st8 [buf]=r0,8 )	// 8-byte aligned
+(p6)	adds len=-8,len;;
+	shr.u cnt=len,4		// number of 128-bit (2x64bit) words
+	;;
+	cmp.eq p6,p0=r0,cnt
+	adds tmp=-1,cnt
+(p6)	br.cond.dpnt .dotail		// we have less than 16 bytes left
+	;;
+	adds buf2=8,buf			// setup second base pointer
+	mov ar.lc=tmp
+	;;
+
+	//
+	// 16bytes/iteration core loop
+	//
+	// The second store can never generate a fault because
+	// we come into the loop only when we are 16-byte aligned.
+	// This means that if we cross a page then it will always be
+	// in the first store and never in the second.
+	//
+	//
+	// We need to keep track of the remaining length. A possible (optimistic)
+	// way would be to use ar.lc and derive how many byte were left by
+	// doing : left= 16*ar.lc + 16.  this would avoid the addition at
+	// every iteration.
+	// However we need to keep the synchronization point. A template
+	// M;;MB does not exist and thus we can keep the addition at no
+	// extra cycle cost (use a nop slot anyway). It also simplifies the
+	// (unlikely)  error recovery code
+	//
+
+2:	EX(.Lexit3, st8 [buf]=r0,16 )
+	;;				// needed to get len correct when error
+	st8 [buf2]=r0,16
+	adds len=-16,len
+	br.cloop.dptk 2b
+	;;
+	mov ar.lc=saved_lc
+	//
+	// tail correction based on len only
+	//
+	// We alternate the use of len3,len2 to allow parallelism and correct
+	// error handling. We also reuse p6/p7 to return correct value.
+	// The addition of len2/len3 does not cost anything more compared to
+	// the regular memset as we had empty slots.
+	//
+.dotail:
+	mov len2=len			// for parallelization of error handling
+	mov len3=len
+	tbit.nz p6,p0=len,3
+	;;
+	EX( .Lexit2, (p6) st8 [buf]=r0,8 )	// at least 8 bytes
+(p6)	adds len3=-8,len2
+	tbit.nz p7,p6=len,2
+	;;
+	EX( .Lexit2, (p7) st4 [buf]=r0,4 )	// at least 4 bytes
+(p7)	adds len2=-4,len3
+	tbit.nz p6,p7=len,1
+	;;
+	EX( .Lexit2, (p6) st2 [buf]=r0,2 )	// at least 2 bytes
+(p6)	adds len3=-2,len2
+	tbit.nz p7,p6=len,0
+	;;
+	EX( .Lexit2, (p7) st1 [buf]=r0 )	// only 1 byte left
+	mov ret0=r0				// success
+	br.ret.sptk.many rp			// end of most likely path
+
+	//
+	// Outlined error handling code
+	//
+
+	//
+	// .Lexit3: comes from core loop, need restore pr/lc
+	//	    len contains bytes left
+	//
+	//
+	// .Lexit2:
+	//	if p6 -> coming from st8 or st2 : len2 contains what's left
+	//	if p7 -> coming from st4 or st1 : len3 contains what's left
+	// We must restore lc/pr even though might not have been used.
+.Lexit2:
+	.pred.rel "mutex", p6, p7
+(p6)	mov len=len2
+(p7)	mov len=len3
+	;;
+	//
+	// .Lexit4: comes from head, need not restore pr/lc
+	//	    len contains bytes left
+	//
+.Lexit3:
+	mov ret0=len
+	mov ar.lc=saved_lc
+	br.ret.sptk.many rp
+END(__do_clear_user)
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index f78a930e92..a0c1a24a4c 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2381,6 +2381,96 @@ static enum hvm_copy_result __hvm_copy(
     return HVMCOPY_okay;
 }
 
+static enum hvm_copy_result __hvm_clear(paddr_t addr, int size)
+{
+    struct vcpu *curr = current;
+    unsigned long gfn, mfn;
+    p2m_type_t p2mt;
+    char *p;
+    int count, todo = size;
+    uint32_t pfec = PFEC_page_present | PFEC_write_access;
+
+    /*
+     * XXX Disable for 4.1.0: PV-on-HVM drivers will do grant-table ops
+     * such as query_size. Grant-table code currently does copy_to/from_guest
+     * accesses under the big per-domain lock, which this test would disallow.
+     * The test is not needed until we implement sleeping-on-waitqueue when
+     * we access a paged-out frame, and that's post 4.1.0 now.
+     */
+#if 0
+    /*
+     * If the required guest memory is paged out, this function may sleep.
+     * Hence we bail immediately if called from atomic context.
+     */
+    if ( in_atomic() )
+        return HVMCOPY_unhandleable;
+#endif
+
+    while ( todo > 0 )
+    {
+        count = min_t(int, PAGE_SIZE - (addr & ~PAGE_MASK), todo);
+
+        gfn = paging_gva_to_gfn(curr, addr, &pfec);
+        if ( gfn == INVALID_GFN )
+        {
+            if ( pfec == PFEC_page_paged )
+                return HVMCOPY_gfn_paged_out;
+            if ( pfec == PFEC_page_shared )
+                return HVMCOPY_gfn_shared;
+            return HVMCOPY_bad_gva_to_gfn;
+        }
+
+        mfn = mfn_x(get_gfn_unshare(curr->domain, gfn, &p2mt));
+
+        if ( p2m_is_paging(p2mt) )
+        {
+            p2m_mem_paging_populate(curr->domain, gfn);
+            put_gfn(curr->domain, gfn);
+            return HVMCOPY_gfn_paged_out;
+        }
+        if ( p2m_is_shared(p2mt) )
+        {
+            put_gfn(curr->domain, gfn);
+            return HVMCOPY_gfn_shared;
+        }
+        if ( p2m_is_grant(p2mt) )
+        {
+            put_gfn(curr->domain, gfn);
+            return HVMCOPY_unhandleable;
+        }
+        if ( !p2m_is_ram(p2mt) )
+        {
+            put_gfn(curr->domain, gfn);
+            return HVMCOPY_bad_gfn_to_mfn;
+        }
+        ASSERT(mfn_valid(mfn));
+
+        p = (char *)map_domain_page(mfn) + (addr & ~PAGE_MASK);
+
+        if ( p2mt == p2m_ram_ro )
+        {
+            static unsigned long lastpage;
+            if ( xchg(&lastpage, gfn) != gfn )
+                gdprintk(XENLOG_DEBUG, "guest attempted write to read-only"
+                        " memory page. gfn=%#lx, mfn=%#lx\n",
+                        gfn, mfn);
+        }
+        else
+        {
+            memset(p, 0x00, count);
+            paging_mark_dirty(curr->domain, mfn);
+        }
+
+        unmap_domain_page(p);
+
+        addr += count;
+        todo -= count;
+        put_gfn(curr->domain, gfn);
+    }
+
+    return HVMCOPY_okay;
+}
+
 enum hvm_copy_result hvm_copy_to_guest_phys(
     paddr_t paddr, void *buf, int size)
 {
@@ -2467,6 +2557,23 @@ unsigned long copy_to_user_hvm(void *to, const void *from, unsigned int len)
     return rc ? len : 0; /* fake a copy_to_user() return code */
 }
 
+unsigned long clear_user_hvm(void *to, unsigned int len)
+{
+    int rc;
+
+#ifdef __x86_64__
+    if ( !current->arch.hvm_vcpu.hcall_64bit &&
+         is_compat_arg_xlat_range(to, len) )
+    {
+        memset(to, 0x00, len);
+        return 0;
+    }
+#endif
+
+    rc = __hvm_clear((unsigned long)to, len);
+    return rc ? len : 0; /* fake a copy_to_user() return code */
+}
+
 unsigned long copy_from_user_hvm(void *to, const void *from, unsigned len)
 {
     int rc;
diff --git a/xen/arch/x86/usercopy.c b/xen/arch/x86/usercopy.c
index d88e635bb0..47dadaeaee 100644
--- a/xen/arch/x86/usercopy.c
+++ b/xen/arch/x86/usercopy.c
@@ -110,6 +110,42 @@ copy_to_user(void __user *to, const void *from, unsigned n)
     return n;
 }
 
+#define __do_clear_user(addr,size)					\
+do {									\
+	long __d0;							\
+	__asm__ __volatile__(						\
+		"0:	rep; stosl\n"					\
+		"	movl %2,%0\n"					\
+		"1:	rep; stosb\n"					\
+		"2:\n"							\
+		".section .fixup,\"ax\"\n"				\
+		"3:	lea 0(%2,%0,4),%0\n"				\
+		"	jmp 2b\n"					\
+		".previous\n"						\
+		_ASM_EXTABLE(0b,3b)					\
+		_ASM_EXTABLE(1b,2b)					\
+		: "=&c"(size), "=&D" (__d0)				\
+		: "r"(size & 3), "0"(size / 4), "1"((long)addr), "a"(0));	\
+} while (0)
+
+/**
+ * clear_user: - Zero a block of memory in user space.
+ * @to:   Destination address, in user space.
+ * @n:    Number of bytes to zero.
+ *
+ * Zero a block of memory in user space.
+ *
+ * Returns number of bytes that could not be cleared.
+ * On success, this will be zero.
+ */
+unsigned long
+clear_user(void __user *to, unsigned n)
+{
+	if ( access_ok(to, n) )
+		__do_clear_user(to, n);
+	return n;
+}
+
 /**
  * copy_from_user: - Copy a block of data from user space.
  * @to:   Destination address, in kernel space.
diff --git a/xen/common/xencomm.c b/xen/common/xencomm.c
index 247545494d..9f6c1c5e9e 100644
--- a/xen/common/xencomm.c
+++ b/xen/common/xencomm.c
@@ -414,6 +414,117 @@ out:
     return n - from_pos;
 }
 
+static int
+xencomm_clear_chunk(
+    unsigned long paddr, unsigned int  len)
+{
+    struct page_info *page;
+    int res;
+
+    do {
+        res = xencomm_get_page(paddr, &page);
+    } while ( res == -EAGAIN );
+
+    if ( res )
+        return res;
+
+    memset(xencomm_vaddr(paddr, page), 0x00, len);
+    xencomm_mark_dirty((unsigned long)xencomm_vaddr(paddr, page), len);
+    put_page(page);
+
+    return 0;
+}
+
+static unsigned long
+xencomm_inline_clear_guest(
+    void *to, unsigned int n, unsigned int skip)
+{
+    unsigned long dest_paddr = xencomm_inline_addr(to) + skip;
+
+    while ( n > 0 )
+    {
+        unsigned int chunksz, bytes;
+
+        chunksz = PAGE_SIZE - (dest_paddr % PAGE_SIZE);
+        bytes   = min(chunksz, n);
+
+        if ( xencomm_clear_chunk(dest_paddr, bytes) )
+            return n;
+        dest_paddr += bytes;
+        n -= bytes;
+    }
+
+    /* Always successful. */
+    return 0;
+}
+
+/**
+ * xencomm_clear_guest: Clear a block of data in domain space.
+ * @to:     Physical address to xencomm buffer descriptor.
+ * @n:      Number of bytes to copy.
+ * @skip: Number of bytes from the start to skip.
+ *
+ * Clear domain data
+ *
+ * Returns number of bytes that could not be cleared
+ * On success, this will be zero.
+ */
+unsigned long
+xencomm_clear_guest(
+    void *to, unsigned int n, unsigned int skip)
+{
+    struct xencomm_ctxt ctxt;
+    unsigned int from_pos = 0;
+    unsigned int to_pos = 0;
+    unsigned int i = 0;
+
+    if ( xencomm_is_inline(to) )
+        return xencomm_inline_clear_guest(to, n, skip);
+
+    if ( xencomm_ctxt_init(to, &ctxt) )
+        return n;
+
+    /* Iterate through the descriptor, copying up to a page at a time */
+    while ( (from_pos < n) && (i < xencomm_ctxt_nr_addrs(&ctxt)) )
+    {
+        unsigned long dest_paddr;
+        unsigned int pgoffset, chunksz, chunk_skip;
+
+        if ( xencomm_ctxt_next(&ctxt, i) )
+            goto out;
+        dest_paddr = *xencomm_ctxt_address(&ctxt);
+        if ( dest_paddr == XENCOMM_INVALID )
+        {
+            i++;
+            continue;
+        }
+
+        pgoffset = dest_paddr % PAGE_SIZE;
+        chunksz = PAGE_SIZE - pgoffset;
+
+        chunk_skip = min(chunksz, skip);
+        to_pos += chunk_skip;
+        chunksz -= chunk_skip;
+        skip -= chunk_skip;
+
+        if ( skip == 0 && chunksz > 0 )
+        {
+            unsigned int bytes = min(chunksz, n - from_pos);
+
+            if ( xencomm_clear_chunk(dest_paddr + chunk_skip, bytes) )
+                goto out;
+            from_pos += bytes;
+            to_pos += bytes;
+        }
+
+        i++;
+    }
+
+out:
+    xencomm_ctxt_done(&ctxt);
+    return n - from_pos;
+}
+
 static int xencomm_inline_add_offset(void **handle, unsigned int bytes)
 {
     *handle += bytes;
diff --git a/xen/include/asm-ia64/uaccess.h b/xen/include/asm-ia64/uaccess.h
index 32ef4151df..2ececb1046 100644
--- a/xen/include/asm-ia64/uaccess.h
+++ b/xen/include/asm-ia64/uaccess.h
@@ -236,6 +236,18 @@ __copy_from_user (void *to, const void __user *from, unsigned long count)
 	__cu_len;									\
 })
 
+extern unsigned long __do_clear_user (void __user * to, unsigned long count);
+
+#define clear_user(to, n)					\
+({								\
+	void __user *__cu_to = (to);							\
+	long __cu_len = (n);								\
+											\
+	if (__access_ok(__cu_to))							\
+		__cu_len = __do_clear_user(__cu_to, __cu_len);	\
+	__cu_len;						\
+})
+
 #define copy_from_user(to, from, n)							\
 ({											\
 	void *__cu_to = (to);								\
diff --git a/xen/include/asm-x86/guest_access.h b/xen/include/asm-x86/guest_access.h
index 99ea64dced..2b429c225f 100644
--- a/xen/include/asm-x86/guest_access.h
+++ b/xen/include/asm-x86/guest_access.h
@@ -21,6 +21,10 @@
     (is_hvm_vcpu(current) ?                     \
      copy_from_user_hvm((dst), (src), (len)) :  \
      copy_from_user((dst), (src), (len)))
+#define raw_clear_guest(dst,  len)              \
+    (is_hvm_vcpu(current) ?                     \
+     clear_user_hvm((dst), (len)) :             \
+     clear_user((dst), (len)))
 #define __raw_copy_to_guest(dst, src, len)      \
     (is_hvm_vcpu(current) ?                     \
      copy_to_user_hvm((dst), (src), (len)) :    \
@@ -29,6 +33,10 @@
     (is_hvm_vcpu(current) ?                     \
      copy_from_user_hvm((dst), (src), (len)) :  \
      __copy_from_user((dst), (src), (len)))
+#define __raw_clear_guest(dst,  len)            \
+    (is_hvm_vcpu(current) ?                     \
+     clear_user_hvm((dst), (len)) :             \
+     clear_user((dst), (len)))
 
 /* Is the guest handle a NULL reference? */
 #define guest_handle_is_null(hnd)        ((hnd).p == NULL)
@@ -69,6 +77,11 @@
     raw_copy_from_guest(_d, _s+(off), sizeof(*_d)*(nr));\
 })
 
+#define clear_guest_offset(hnd, off, nr) ({    \
+    void *_d = (hnd).p;                        \
+    raw_clear_guest(_d+(off), nr);             \
+})
+
 /* Copy sub-field of a structure to guest context via a guest handle. */
 #define copy_field_to_guest(hnd, ptr, field) ({         \
     const typeof(&(ptr)->field) _s = &(ptr)->field;     \
@@ -110,6 +123,11 @@
     __raw_copy_from_guest(_d, _s+(off), sizeof(*_d)*(nr));\
 })
 
+#define __clear_guest_offset(hnd, off, nr) ({    \
+    void *_d = (hnd).p;                          \
+    __raw_clear_guest(_d+(off), nr);             \
+})
+
 #define __copy_field_to_guest(hnd, ptr, field) ({       \
     const typeof(&(ptr)->field) _s = &(ptr)->field;     \
     void *_d = &(hnd).p->field;                         \
diff --git a/xen/include/asm-x86/hvm/guest_access.h b/xen/include/asm-x86/hvm/guest_access.h
index 7a89e81536..b92dbe90d5 100644
--- a/xen/include/asm-x86/hvm/guest_access.h
+++ b/xen/include/asm-x86/hvm/guest_access.h
@@ -2,6 +2,7 @@
 #define __ASM_X86_HVM_GUEST_ACCESS_H__
 
 unsigned long copy_to_user_hvm(void *to, const void *from, unsigned len);
+unsigned long clear_user_hvm(void *to, unsigned int len);
 unsigned long copy_from_user_hvm(void *to, const void *from, unsigned len);
 
 #endif /* __ASM_X86_HVM_GUEST_ACCESS_H__ */
diff --git a/xen/include/asm-x86/uaccess.h b/xen/include/asm-x86/uaccess.h
index e3e541b306..d6f44584f6 100644
--- a/xen/include/asm-x86/uaccess.h
+++ b/xen/include/asm-x86/uaccess.h
@@ -16,6 +16,7 @@
 #endif
 
 unsigned long copy_to_user(void *to, const void *from, unsigned len);
+unsigned long clear_user(void *to, unsigned len);
 unsigned long copy_from_user(void *to, const void *from, unsigned len);
 /* Handles exceptions in both to and from, but doesn't do access_ok */
 unsigned long __copy_to_user_ll(void *to, const void *from, unsigned n);
diff --git a/xen/include/xen/guest_access.h b/xen/include/xen/guest_access.h
index 0b9fb071a4..373454ec83 100644
--- a/xen/include/xen/guest_access.h
+++ b/xen/include/xen/guest_access.h
@@ -15,10 +15,16 @@
 #define copy_from_guest(ptr, hnd, nr)                   \
     copy_from_guest_offset(ptr, hnd, 0, nr)
 
+#define clear_guest(hnd, nr)                            \
+    clear_guest_offset(hnd, 0, nr)
+
 #define __copy_to_guest(hnd, ptr, nr)                   \
     __copy_to_guest_offset(hnd, 0, ptr, nr)
 
 #define __copy_from_guest(ptr, hnd, nr)                 \
     __copy_from_guest_offset(ptr, hnd, 0, nr)
 
+#define __clear_guest(hnd, nr)                          \
+    __clear_guest_offset(hnd, 0, nr)
+
 #endif /* __XEN_GUEST_ACCESS_H__ */
diff --git a/xen/include/xen/xencomm.h b/xen/include/xen/xencomm.h
index bce2ca720d..730da7c1f2 100644
--- a/xen/include/xen/xencomm.h
+++ b/xen/include/xen/xencomm.h
@@ -27,6 +27,8 @@ unsigned long xencomm_copy_to_guest(
     void *to, const void *from, unsigned int len, unsigned int skip); 
 unsigned long xencomm_copy_from_guest(
     void *to, const void *from, unsigned int len, unsigned int skip); 
+unsigned long xencomm_clear_guest(
+    void *to, unsigned int n, unsigned int skip);
 int xencomm_add_offset(void **handle, unsigned int bytes);
 int xencomm_handle_is_null(void *ptr);
 
@@ -41,6 +43,16 @@ static inline unsigned long xencomm_inline_addr(const void *handle)
     return (unsigned long)handle & ~XENCOMM_INLINE_FLAG;
 }
 
+#define raw_copy_to_guest(dst, src, len)       \
+    xencomm_copy_to_guest(dst, src, len, 0)
+#define raw_copy_from_guest(dst, src, len)     \
+    xencomm_copy_from_guest(dst, src, nr, 0)
+#define raw_clear_guest(dst, len)              \
+    xencomm_clear_guest(dst, len, 0)
+#define __raw_copy_to_guest raw_copy_to_guest
+#define __raw_copy_from_guest raw_copy_from_guest
+#define __raw_clear_guest raw_clear_guest
+
 /* Is the guest handle a NULL reference? */
 #define guest_handle_is_null(hnd) \
     ((hnd).p == NULL || xencomm_handle_is_null((hnd).p))
@@ -82,6 +94,13 @@ static inline unsigned long xencomm_inline_addr(const void *handle)
 #define copy_from_guest_offset(ptr, hnd, idx, nr) \
     __copy_from_guest_offset(ptr, hnd, idx, nr)
 
+/*
+ * Clear an array of objects in guest context via a guest handle.
+ * Optionally specify an offset into the guest array.
+ */
+#define clear_guest_offset(hnd, idx, nr) \
+    __clear_guest_offset(hnd, idx, nr)
+
 /* Copy sub-field of a structure from guest context via a guest handle. */
 #define copy_field_from_guest(ptr, hnd, field) \
     __copy_field_from_guest(ptr, hnd, field)
@@ -115,6 +134,11 @@ static inline unsigned long xencomm_inline_addr(const void *handle)
     xencomm_copy_from_guest(_d, _s, sizeof(*_d), _off);             \
 })
 
+#define __clear_guest_offset(hnd, idx, nr) ({                \
+    void *_d = (hnd).p;                                             \
+    xencomm_clear_guest(_d, nr, idx); \
+})
+
 #ifdef CONFIG_XENCOMM_MARK_DIRTY
 extern void xencomm_mark_dirty(unsigned long addr, unsigned int len);
 #else