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, 5 insertions, 1 deletions

diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index d52dbd715c..608accfe48 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -204,9 +204,13 @@ int xstate_alloc_save_area(struct vcpu *v)
     if ( save_area == NULL )
         return -ENOMEM;
 
+    /*
+     * Set the memory image to default values, but don't force the context
+     * to be loaded from memory (i.e. keep save_area->xsave_hdr.xstate_bv
+     * clear).
+     */
     save_area->fpu_sse.fcw = FCW_DEFAULT;
     save_area->fpu_sse.mxcsr = MXCSR_DEFAULT;
-    save_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
 
     v->arch.xsave_area = save_area;
     v->arch.xcr0 = XSTATE_FP_SSE;