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

Marcelo Tosatti
2012-12-25T21:37:18Z

On Tue, Dec 25, 2012 at 02:48:08PM +0200, Gleb Natapov wrote:
> 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).
> True. Marcelo what failure do you see without the patch?
>
> --
> Gleb.

On transition _to_ real mode, linearize fails due to CPL checks
(FreeBSD). I'll resend the patch with use of cache for
VMREAD(GUEST_RFLAGS), which is already implemented.