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 <[email protected]> --- arch/x86/kvm/vmx.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 4f3d6ac..939e4a0 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -3316,7 +3316,7 @@ 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; -- 1.7.11.3 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
