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

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Date:   Mon, 23 Sep 2019 18:48:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, x86@...nel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH 2/3] KVM: x86: hyper-v: set NoNonArchitecturalCoreSharing
 CPUID bit when SMT is impossible

On 23/09/19 17:37, Peter Zijlstra wrote:
>> This patch reports NoNonArchitecturalCoreSharing bit in to userspace in the
>> first case. The second case is outside of KVM's domain of responsibility
>> (as vCPU pinning is actually done by someone who manages KVM's userspace -
>> e.g. libvirt pinning QEMU threads).
> This is purely about guest<->guest MDS, right? Ie. not worse than actual
> hardware.

Even within the same guest.  If vCPU 1 is on virtual core 1 and vCPU 2
is on virtual core 2, but they can share the same physical core, core
scheduling in the guest can do nothing about it.

Paolo