Re: [PATCH] KVM: x86: Set BHI_NO in guest when host is not affected by BHI

[Chao Gao]

On Thu, Apr 11, 2024 at 05:20:30PM +0200, Paolo Bonzini wrote:
>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.

This looks a good solution.

We can also introduce a new bit indicating the effectiveness of the short
BHB-clearing sequence. KVM advertises this bit for all pre-SPR/ADL parts.
Only if the bit is 1, guests will use the short BHB-clearing sequence.
Otherwise guests should use the long sequence. In a mixed migration pool,
the VMM shouldn't expose the bit to guests.

>
>Paolo
>
>