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KVM: VMX: Allow vm86 virtualization of big real mode

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Usually, big real mode uses large (4GB) segments.  Currently we don't
virtualize this; if any segment has a limit other than 0xffff, we emulate.
But if we set the vmx-visible limit to 0xffff, we can use vm86 to virtualize
real mode; if an access overruns the segment limit, the guest will #GP, which
we will trap and forward to the emulator.  This results in significantly
faster execution, and less risk of hitting an unemulated instruction.

If the limit is less than 0xffff, we retain the existing behaviour.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Avi Kivity 2012-08-21 17:07:03 +03:00 committed by Marcelo Tosatti
parent 495e116684
commit e2a610d7fc
1 changed files with 1 additions and 1 deletions
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arch/x86/kvm/vmx.c
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@ -3315,7 +3315,7 @@ static bool rmode_segment_valid(struct kvm_vcpu *vcpu, int seg)
if (var.base != (var.selector << 4))
return false;
if (var.limit != 0xffff)
if (var.limit < 0xffff)
return false;
if ((ar | (3 << AR_DPL_SHIFT)) != 0xf3)
return false;