Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Pawan Gupta]

On Mon, Mar 09, 2026 at 04:05:55PM -0700, Jim Mattson wrote:
> On Mon, Mar 9, 2026 at 3:29 PM Pawan Gupta
> <pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
> > > Is it reasonable to assume that without the presence of BHI_CTRL, the
> > > non-Linux OS we've been discussing will (ironically) only use the long
> > > sequence if the hypervisor advertises BHB_CLEAR_SEQ_S_SUPPORT? That
> > > is, without BHB_CLEAR_SEQ_S_SUPPORT, does it assume the short sequence
> > > is adequate?
> >
> > I don't know. But, it doesn't seem logical to assume short sequence is
> > adequate when the guest can't ensure that VMM would do BHI_DIS_S for it. It
> > should be the other way around.
> 
> Assuming BHI_NO is clear...
> 
> If the hypervisor offers to enable BHI_DIS_S for you, then the
> migration pool may contain SPR+, so you need the long sequence if
> you're going to clear the BHB in software rather than accept the
> hypervisor's offer.
> You are saying that if the hypervisor does not offer to enable
> BHI_DIS_S for you, then you know nothing, so you need the long
> sequence.

This is all when MSR_VIRTUAL_MITIGATION_* is enumerated by the VMM.

> How would a guest that refuses BHI_DIS_S ever be able to use the short sequence?

Without those MSRs a guest could ignore the migration case, and use the
short sequence.