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

Pawan Gupta
2026-04-04T00:21:58Z

On Fri, Apr 03, 2026 at 04:39:54PM -0700, Jim Mattson wrote:
> > Since cloud providers have greater control over userspace, the decision to
> > use BHI_DIS_S or not can be left to them. KVM would simply follow what it
> > is asked to do by the userspace.
>
> I feel like we've gone over this before, but if userspace tells KVM
> not to enable BHI_DIS_S, how do we inform Windows that it needs to do
> the longer clearing sequence, despite the fact that the virtual CPU is
> masquerading as Ice Lake?

IMO, if an OS is allergic to a hardware mitigation, and is also aware that
it is virtualized, it should default to a sw mitigation that works everywhere.

> I don't think the virtual mitigation MSRs address that issue.

Virtual mitigation MSRs are meant to inform the VMM about the guest
mitigation. Even if there was a way to tell the guest that it needs to use
a different mitigation, it seems unrealistic for a guest to change its
mitigation post-migration.