Re: [PATCH v9 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Jim Mattson]

On Tue, Apr 7, 2026 at 3:27 PM Pawan Gupta
<pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Apr 07, 2026 at 01:53:24PM -0700, Jim Mattson wrote:
> > On Tue, Apr 7, 2026 at 12:11 PM Pawan Gupta
> > <pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Apr 07, 2026 at 11:40:57AM -0700, Jim Mattson wrote:
> > > > My proposal is as follows:
> > > >
> > > > 1. The (advanced) hypervisor can advertise to the guest (via CPUID bit
> > > > or MSR bit) that the short BHB clearing sequence is adequate. This may
> > > > mean either that the VM will only be hosted on pre-Alder Lake hardware
> > > > or that the hypervisor will set BHI_DIS_S behind the back of the
> > > > guest. Presumably, this bit would not be reported if BHI_CTRL is
> > > > advertised to the guest.
> > > > 2. If the guest sees this bit, then it can use the short sequence. If
> > > > it doesn't see this bit, it must use the long sequence.
> > >
> > > Thats a good middle ground. Let me check with folks internally what they
> > > think about defining a new software-only bit.
> > >
> > > Third case, for a guest that doesn't want BHI_DIS_S, userspace should be
> > > allowed to override setting BHI_DIS_S. Then this proposed bit can indicate
> > > that long sequence is required.
> >
> > That case can be handled by the paravirtual mitigation MSRs, right?
>
> Yes. But, that was the part that received the most pushback.

What is your proposed BHI_DIS_S override mechanism, then?