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

[Alexandre Chartre <alexandre.chartre@oracle.com>]

On 4/11/24 15:22, Paolo Bonzini wrote:
> On Thu, Apr 11, 2024 at 11:34 AM Alexandre Chartre
> <alexandre.chartre@oracle.com> wrote:
>>
>> So you mean we can't set ARCH_CAP_BHI_NO for the guest because we don't know
>> if the guest will run the (other) existing mitigations which are believed to
>> suffice to mitigate BHI?
>>
>> The problem is that we can end up with a guest running extra BHI mitigations
>> while this is not needed. Could we inform the guest that eIBRS is not available
>> on the system so a Linux guest doesn't run with extra BHI mitigations?
> 
> 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. If you hide bit 1 of
> MSR_IA32_ARCH_CAPABILITIES from the guest, it may decide to disable
> it. But if the guest decides to use eIBRS, I think it should use
> mitigations even if the host doesn't.

The problem is not on servers which have eIBRS, but on servers which don't.

If there is no eIBRS on the server, then the guest doesn't know if there is
effectively no eIBRS on the server or if eIBRS is hidden by the virtualization
so it applies the BHI mitigation even when that's not needed (i.e. when eIBRS
is effectively not present the server).

> It's a different story if the host isn't susceptible altogether. The
> ARCH_CAP_BHI_NO bit *can* be set if the processor doesn't have the bug
> at all, which would be true if cpu_matches(cpu_vuln_whitelist,
> NO_BHI). I would apply a patch to do that.
> 

Right. I have just suggested to enumerate cpus which have eIBRS with NO_BHI,
but we need would that precise list of cpus.

alex.