x86/xsave: adjust state management

The initial state for a vCPU is using default values, so there's no
need to force the XRSTOR to read the state from memory. This saves a
couple of thousand restores from memory just during boot of Linux on
my Sandy Bridge system (I didn't try to make further measurements).

The above requires that arch_set_info_guest() updates the state flags
in the save area when valid floating point state got passed in, but
that would really have been needed even before in case XSAVE{,OPT}
decided to clear one or both of the FP and SSE bits.

Furthermore, hvm_vcpu_reset_state() shouldn't just clear out the FPU/
SSE area, but needs to re-initialized MXCSR and FCW.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 43b6d052ba..1fcaed0b83 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3466,6 +3466,7 @@ void hvm_vcpu_reset_state(struct vcpu *v, uint16_t cs, uint16_t ip)
 {
     struct domain *d = v->domain;
     struct segment_register reg;
+    typeof(v->arch.xsave_area->fpu_sse) *fpu_ctxt = v->arch.fpu_ctxt;
 
     domain_lock(d);
 
@@ -3479,7 +3480,12 @@ void hvm_vcpu_reset_state(struct vcpu *v, uint16_t cs, uint16_t ip)
         v->arch.guest_table = pagetable_null();
     }
 
-    memset(v->arch.fpu_ctxt, 0, sizeof(v->arch.xsave_area->fpu_sse));
+    memset(fpu_ctxt, 0, sizeof(*fpu_ctxt));
+    fpu_ctxt->fcw = FCW_RESET;
+    fpu_ctxt->mxcsr = MXCSR_DEFAULT;
+    if ( v->arch.xsave_area )
+        v->arch.xsave_area->xsave_hdr.xstate_bv = XSTATE_FP;
+
     v->arch.vgc_flags = VGCF_online;
     memset(&v->arch.user_regs, 0, sizeof(v->arch.user_regs));
     v->arch.user_regs.eflags = 2;