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

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On 4/11/24 13:14, Chao Gao wrote:
The problem is that we can end up with a guest running extra BHI
while this is not needed. Could we inform the guest that eIBRS is not
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.


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