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 17:37:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        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 Mon, Sep 16, 2019 at 06:22:57PM +0200, Vitaly Kuznetsov 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).
> 
> 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.