Re: KVM: VMX: fix incorrect cached cpl value with real/v8086 modes

Marcelo Tosatti
2013-01-01T23:42:40Z

On Sat, Dec 22, 2012 at 02:31:10PM +0200, Avi Kivity wrote:
> On Wed, Dec 19, 2012 at 3:29 PM, Marcelo Tosatti <mtosatti@redhat.com>wrote:
>
> >
> >
> > CPL is always 0 when in real mode, and always 3 when virtual 8086 mode.
> >
> > Using values other than those can cause failures on operations that check
> > CPL.
> >
> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> >
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index a4ecf7c..3abe433 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -3215,13 +3215,6 @@ static u64 vmx_get_segment_base(struct kvm_vcpu
> > *vcpu, int seg)
> >
> > static int __vmx_get_cpl(struct kvm_vcpu *vcpu)
> > {
> > - if (!is_protmode(vcpu))
> > - return 0;
> > -
> > - if (!is_long_mode(vcpu)
> > - && (kvm_get_rflags(vcpu) & X86_EFLAGS_VM)) /* if virtual 8086
> > */
> > - return 3;
> > -
> > return vmx_read_guest_seg_selector(to_vmx(vcpu), VCPU_SREG_CS) & 3;
> > }
> >
> > @@ -3229,6 +3222,13 @@ static int vmx_get_cpl(struct kvm_vcpu *vcpu)
> > {
> > struct vcpu_vmx *vmx = to_vmx(vcpu);
> >
> > + if (!is_protmode(vcpu))
> > + return 0;
> > +
> > + if (!is_long_mode(vcpu)
> > + && (kvm_get_rflags(vcpu) & X86_EFLAGS_VM)) /* if virtual 8086
> > */
> > + return 3;
> > +
> > /*
> > * If we enter real mode with cs.sel & 3 != 0, the normal CPL
> > calculations
> > * fail; use the cache instead.
> >
>
>
> This undoes the cache, now every vmx_get_cpl() in protected mode has to
> VMREAD(GUEST_RFLAGS).

Yes, but GUEST_RFLAGS reads are cached. So its read once per exit.