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

Gleb Natapov
2012-12-26T05:25:51Z

On Tue, Dec 25, 2012 at 07:37:10PM -0200, Marcelo Tosatti wrote:
> 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.
I am curious does it still fails with all my vmx patches applied too?
The question is how does it happen that we enter real mode while cache
is set to 3. It should never be 3 during boot since boot process never
enters the userspace.

--
Gleb.