linux-kernel - Re: [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX

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Message-ID: <YBgugM03fsEiOxz1@google.com>
Date:   Mon, 1 Feb 2021 08:38:24 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even
 if tsx=off

On Mon, Feb 01, 2021, Paolo Bonzini wrote:
> On 01/02/21 09:46, Paolo Bonzini wrote:
> > > 
> > > This comment be updated to call out the new TSX_CTRL behavior.
> > > 
> > >     /*
> > >      * On TAA affected systems:
> > >      *      - nothing to do if TSX is disabled on the host.
> > >      *      - we emulate TSX_CTRL if present on the host.
> > >      *      This lets the guest use VERW to clear CPU buffers.
> > >      */
> > 
> > Ok.
> 
> Hmm, but the comment is even more accurate now than before, isn't it? It
> said nothing about hiding TSX_CTRL, so now it matches the code below.

Ha, that is technically true.  But it says "nothing to do..." and then clears a
flag.  The other interpretation of "nothing to do... at runtime" is also wrong
as KVM emulates the MSR as a nop.

I guess I just find the whole comment more confusing than the code itself.