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

[Jim Mattson]

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()?

> +       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
>
>