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Subject: Re: [PATCH 2/3] KVM: x86: hyper-v: set NoNonArchitecturalCoreSharing
 CPUID bit when SMT is impossible
To:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Jim Mattson <jmattson@google.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-hyperv@vger.kernel.org,
        the arch/x86 maintainers <x86@kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Michael Kelley <mikelley@microsoft.com>,
        Roman Kagan <rkagan@virtuozzo.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Tue, 17 Sep 2019 16:08:32 +0200
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On 17/09/19 11:33, Vitaly Kuznetsov wrote:
> 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?

Yes, that's a good idea.

Paolo