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>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 4811d91759a11d123e30053415c49595b6cdc4f7..fd21eb454663ca75e8f3c1923b5e430d73956379 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -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;