linux-kernel - Re: [PATCH v3 19/34] KVM: nVMX: hyper-v: Enable L2 TLB flush

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Message-ID: <YoKxDbWDvKmaLiC3@google.com>
Date:   Mon, 16 May 2022 20:16:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 19/34] KVM: nVMX: hyper-v: Enable L2 TLB flush

On Wed, May 11, 2022, Maxim Levitsky wrote:
> On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
> > +/*
> > + * Note, Hyper-V isn't actually stealing bit 28 from Intel, just abusing it by
> > + * pairing it with architecturally impossible exit reasons.  Bit 28 is set only
> > + * on SMI exits to a SMI transfer monitor (STM) and if and only if a MTF VM-Exit
> > + * is pending.  I.e. it will never be set by hardware for non-SMI exits (there
> > + * are only three), nor will it ever be set unless the VMM is an STM.
> 
> I am sure that this will backfire this way or another. Their fault though...

Heh, that was my initial reaction too, but after working through the architecture
I gotta hand it to the Hyper-V folks, it's very clever :-)  And if we ever need a
synthetic exit reason for PV KVM... :-)