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

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Date:   Thu, 14 Sep 2023 11:24:35 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        David Woodhouse <dwmw2@...radead.org>
Cc:     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 13/9/2023 11:24 pm, David Woodhouse wrote:
> 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.

If we combine the v5 code diff (the u64 *user_value proposal) with the refined 
changelog in v6,
it seems like we've reached a point of equilibrium on this issue, doesn't it ?

Please let me know you have more concerns.

> 
> 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 :)
>