Messages in this thread |  | | From | Jim Mattson <> | | Date | Mon, 9 Mar 2026 17:08:09 -0700 | | Subject | Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on newer CPUs |
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On Mon, Mar 9, 2026 at 5:00 PM Pawan Gupta <pawan.kumar.gupta@linux.intel.com> wrote: > > 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@linux.intel.com> 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.
Ah. So, a hypervisor only advertises MSR_VIRTUAL_MITIGATION_* when the migration pool spans the change in BHB size?
Why not just declare that explicitly? This whole mechanism seems to be an exercise in obfuscation.
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