Re: [PATCH 2/3] KVM: x86: hyper-v: set NoNonArchitecturalCoreSharing CPUID bit when SMT is impossible

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jim Mattson <jmattson@google.com> writes:

> On Mon, Sep 16, 2019 at 9:23 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>
>> Hyper-V 2019 doesn't expose MD_CLEAR CPUID bit to guests when it cannot
>> guarantee that two virtual processors won't end up running on sibling SMT
>> threads without knowing about it. This is done as an optimization as in
>> this case there is nothing the guest can do to protect itself against MDS
>> and issuing additional flush requests is just pointless. On bare metal the
>> topology is known, however, when Hyper-V is running nested (e.g. on top of
>> KVM) it needs an additional piece of information: a confirmation that the
>> exposed topology (wrt vCPU placement on different SMT threads) is
>> trustworthy.
>>
>> NoNonArchitecturalCoreSharing (CPUID 0x40000004 EAX bit 18) is described in
>> TLFS as follows: "Indicates that a virtual processor will never share a
>> physical core with another virtual processor, except for virtual processors
>> that are reported as sibling SMT threads." From KVM we can give such
>> guarantee in two cases:
>> - SMT is unsupported or forcefully disabled (just 'disabled' doesn't work
>>  as it can become re-enabled during the lifetime of the guest).
>> - vCPUs are properly pinned so the scheduler won't put them on sibling
>> SMT threads (when they're not reported as such).
>
> That's a nice bit of information. Have you considered a mechanism for
> communicating this information to kvm guests in a way that doesn't
> require Hyper-V enlightenments?
>

(I haven't put much thought in this) but can we re-use MD_CLEAR CPUID
bit for that? Like if the hypervisor can't guarantee usefulness
(e.g. when two random vCPUs can be put on sibling SMT threads) of
flushing, is there any reason to still make the guest think the feature
is there?

-- 
Vitaly