Commit e2a610d7 authored Aug 21, 2012 by Avi Kivity Committed by Marcelo Tosatti Aug 27, 2012

KVM: VMX: Allow vm86 virtualization of big real mode

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>

parent 495e1166

arch/x86/kvm/vmx.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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;

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