linux-kernel - Re: [PATCH v4] KVM: x86/tsc: Don't sync user changes to TSC with KVM-initiated change

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Date:   Wed, 13 Sep 2023 17:24:35 +0200
From:   David Woodhouse <dwmw2@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Like Xu <like.xu.linux@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Oliver Upton <oliver.upton@...ux.dev>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] KVM: x86/tsc: Don't sync user changes to TSC with
 KVM-initiated change

On Wed, 2023-09-13 at 15:15 +0000, Sean Christopherson wrote:
> > > e.g. if userspace writes '0' immediately after creating, and then later writes a
> > > small delta, the v6 code wouldn't trigger synchronization because "user_set_tsc"
> > > would be left unseft by the write of '0'.
> > 
> > True, but that's the existing behaviour,
> 
> No?  The existing code will fall into the "conditionally sync" logic for any
> non-zero value.

Yeah, OK. This isn't one of the cases we set out to deliberately
change, but it would be changed by v6 of the patch, and I suppose
you're right that we should accept a small amount of extra code
complexity just to avoid making any changes we don't *need* to, even
for stupid cases like this.

> I don't care (in the Tommy Lee Jones[*] sense).  All I care about is minimizing
> the probability of breaking userspace, which means making the smallest possible
> change to KVM's ABI.  For me, whether or not userspace is doing something sensible
> doesn't factor into that equation.

Ack.

Although there's a strong argument that adding further warts to an
already fundamentally broken API probably isn't a great idea in the
first place. Just deprecate it and use the saner replacement API...
which I just realised we don't have (qv). Ooops :)

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