x86: Special case __HYPERVISOR_iret rather more when writing hypercall pages

In all cases when a hypercall page is written, __HYPERVISOR_iret is first
written as a regular hypercall, then subsequently rewritten in its special
case.

For VMX and SVM, this means that following the ud2a instruction is 3 bytes of
an imm32 parameter.  For a ring3 kernel, this means that following the syscall
instruction is the second half of 'pop %r11'.

For a ring1 kernel, the iret case ends up as the same number of bytes as the
rest of the hypercalls, but it is pointless writing it twice, and is changed
for consistency.

Therefore, skip the loop iteration which would write the incorrect
__HYPERVISOR_iret hypercall.  This removes junk machine code from the tail and
makes disassemblers rather more happy when looking at the hypercall page.

Also, a miscellaneous whitespace fix in the comment for ring3 kernel.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index eec919aebb..bcd760908e 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -590,6 +590,9 @@ static void hypercall_page_initialise_ring3_kernel(void *hypercall_page)
     /* Fill in all the transfer points with template machine code. */
     for ( i = 0; i < (PAGE_SIZE / 32); i++ )
     {
+        if ( i == __HYPERVISOR_iret )
+            continue;
+
         p = (char *)(hypercall_page + (i * 32));
         *(u8  *)(p+ 0) = 0x51;    /* push %rcx */
         *(u16 *)(p+ 1) = 0x5341;  /* push %r11 */
@@ -602,8 +605,8 @@ static void hypercall_page_initialise_ring3_kernel(void *hypercall_page)
     }
 
     /*
-     * HYPERVISOR_iret is special because it doesn't return and expects a 
-     * special stack frame. Guests jump at this transfer point instead of 
+     * HYPERVISOR_iret is special because it doesn't return and expects a
+     * special stack frame. Guests jump at this transfer point instead of
      * calling it.
      */
     p = (char *)(hypercall_page + (__HYPERVISOR_iret * 32));