Re: [PATCH] KVM: x86: avoid unnecessary XSETBV on guest entry

[Quan Xu]

On 2017/12/13 20:51, Paolo Bonzini wrote:

xsetbv can be expensive when running on nested virtualization, try to
avoid it.

Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
  arch/x86/kvm/x86.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0f82e2cbf64c..daa1918031df 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -702,7 +702,8 @@ static void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
        if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
                        !vcpu->guest_xcr0_loaded) {
                /* kvm_set_xcr() also depends on this */
-               xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
+               if (vcpu->arch.xcr0 != host_xcr0)
+                       xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
                vcpu->guest_xcr0_loaded = 1;
        }
  }

Reviewed-by: Quan Xu <quan....@gmail.com>