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x86, kvm, vmx: Don't set LOAD_IA32_EFER when host and guest match

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There's nothing to switch if the host and guest values are the same.
I am unable to find evidence that this makes any difference
whatsoever.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
[I could see a difference on Nehalem.  From 5 runs:

 userspace exit, guest!=host   12200 11772 12130 12164 12327
 userspace exit, guest=host    11983 11780 11920 11919 12040
 lightweight exit, guest!=host  3214  3220  3238  3218  3337
 lightweight exit, guest=host   3178  3193  3193  3187  3220

 This passes the t-test with 99% confidence for userspace exit,
 98.5% confidence for lightweight exit. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Andy Lutomirski 2014-11-10 11:19:15 -08:00 committed by Paolo Bonzini
parent f6577a5fa1
commit 54b98bff8e
1 changed files with 3 additions and 1 deletions
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4
arch/x86/kvm/vmx.c
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@ -1673,7 +1673,9 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
guest_efer = vmx->vcpu.arch.efer;
if (!(guest_efer & EFER_LMA))
guest_efer &= ~EFER_LME;
add_atomic_switch_msr(vmx, MSR_EFER, guest_efer, host_efer);
if (guest_efer != host_efer)
add_atomic_switch_msr(vmx, MSR_EFER,
guest_efer, host_efer);
return false;
}