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

[Sean Christopherson <seanjc@google.com>]

On Mon, Jun 15, 2026, Jim Mattson wrote:
> On Fri, Jun 12, 2026 at 4:02 PM Sean Christopherson <seanjc@google.com> 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@google.com>
> > ---
> >  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.
	 */

> > +       if (WARN_ON_ONCE(ctxt->ops->set_dr(ctxt, ctxt->modrm_reg, val)))
> >                 return emulate_gp(ctxt, 0);
> >
> >         /* Disable writeback. */
> > --
> > 2.54.0.1136.gdb2ca164c4-goog
> >
> >