On Thu, Apr 11, 2024 at 5:13 PM Alexandre Chartre <alexandre.chartre@xxxxxxxxxx> wrote: > I think that Andrew's concern is that if there is no eIBRS on the host then > we do not set X86_BUG_BHI on the host because we know the kernel which is > running and this kernel has some mitigations (other than the explicit BHI > mitigations) and these mitigations are enough to prevent BHI. But still > the cpu is affected by BHI. Hmm, then I'm confused. It's what I wrote before: "The (Linux or otherwise) guest will make its own determinations as to whether BHI mitigations are necessary. If the guest uses eIBRS, it will run with mitigations" but you said machines without eIBRS are fine. If instead they are only fine _with Linux_, then yeah we cannot set BHI_NO in general. What we can do is define a new bit that is in the KVM leaves. The new bit is effectively !eIBRS, except that it is defined in such a way that, in a mixed migration pool, both eIBRS and the new bit will be 0. Paolo
