Re: [PATCH v3 7/8] KVM: x86: WARN if MOV DR emulation hits a "too late" #GP

[Jim Mattson]

On Mon, Jun 15, 2026 at 12:08 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Mon, Jun 15, 2026, Jim Mattson wrote:
> > On Fri, Jun 12, 2026 at 4:02 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > >
> > > WARN if ->set_dr() => kvm_set_dr() fails when emulating a MOV DR write,
> > > as the emulator _must_ pre-check for #GPs in order to get the event
> > > priority right when emulating MOV DR for L2 on SVM (all exceptions have
> > > higher priority than the instruction intercept).
> > >
> > > Opportunistically update the comment as the blurb about "#UD" being
> > > checked is incomplete and misleading.
> > >
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/emulate.c | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > > index b1799ed01939..e030ef9b9744 100644
> > > --- a/arch/x86/kvm/emulate.c
> > > +++ b/arch/x86/kvm/emulate.c
> > > @@ -3298,8 +3298,12 @@ static int em_dr_write(struct x86_emulate_ctxt *ctxt)
> > >         else
> > >                 val = ctxt->src.val & ~0U;
> > >
> > > -       /* #UD condition is already handled. */
> > > -       if (ctxt->ops->set_dr(ctxt, ctxt->modrm_reg, val))
> > > +       /*
> > > +        * A #GP due to an illegal value should be impossible at this point, as
> > > +        * such #GPs have priority over MOV DR intercepts on SVM, i.e. KVM must
> > > +        * manually check the value *before* emulating the write.
> > > +        */
> >
> > This comment seems confusing to me. Isn't an illegal value impossible
> > simply because we already called check_dr_write()?
>
> Yeah.  I'm trying to explain why KVM WARNs, and also to guard against dropping
> the value checks in check_dr_write() as future "cleanups".
>
> Is this better?
>
>         /*
>          * Yell if the DR write is rejected, as all inputs have been validated.
>          * KVM must manually check for exceptions *before* emulating the write,
>          * as all exceptions have priority over MOV DR intercepts on SVM.
>          */
>
Better, thanks. With that change,

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>