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 13:14, Chao Gao wrote:
>>> 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?
>>
>> Well, that's why Intel specified some MSRs at 0x5000xxxx.
> 
> Yes. But note that there is a subtle difference. Those MSRs are used for guest
> to communicate in-used software mitigations to the host. Such information is
> stable across migration. Here we need the host to communicate that eIBRS isn't
> available to the guest. this isn't stable as the guest may be migrated from
> a host without eIBRS to one with it.
> 
>>
>> Except I don't know anyone currently interested in implementing them,
>> and I'm still not sure if they work correctly for some of the more
>> complicated migration cases.
> 
> Looks you have the same opinion on the Intel-defined virtual MSRs as Sean.
> If we all agree the issue here and the effectivenss problem of the short
> BHB-clearing sequence need to be resolved and don't think the Intel-defined
> virtual MSRs can handle all cases correctly, we have to define a better
> interface through community collaboration as Sean suggested.

Another solution could be to add cpus to cpu_vuln_whitelist with BHI_NO.
(e.g. explicitly add cpus which have eIBRS). That way, the kernel will
figure out the right mitigation on the host and guest.

alex.