linux-kernel - Re: [PATCH v7] KVM: x86/tsc: Don't sync user-written TSC against startup values

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Date:   Mon,  9 Oct 2023 17:44:32 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Like Xu <like.xu.linux@...il.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Oliver Upton <oliver.upton@...ux.dev>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7] KVM: x86/tsc: Don't sync user-written TSC against
 startup values

On Sun, 08 Oct 2023 10:53:35 +0800, Like Xu wrote:
> The legacy API for setting the TSC is fundamentally broken, and only
> allows userspace to set a TSC "now", without any way to account for
> time lost to preemption between the calculation of the value, and the
> kernel eventually handling the ioctl.
> 
> To work around this we have had a hack which, if a TSC is set with a
> value which is within a second's worth of a previous vCPU, assumes that
> userspace actually intended them to be in sync and adjusts the newly-
> written TSC value accordingly.
> 
> [...]

Applied to kvm-x86 misc, thanks!  I massaged away most of the pronouns in the
changelog.  Yes, they bug me that much, and I genuinely had a hard time following
some of the paragraphs even though I already knew what the patch is doing.

Everyone, please take a look and make sure I didn't botch anything.  I tried my
best to keep the existing "voice" and tone of the changelog (sans pronouns
obviously).  I definitely don't want to bikeshed this thing any further.  If
I've learned anything by this patch, it's that the only guaranteed outcome of
changelog-by-committee is that no one will walk away 100% happy :-)

[1/1] KVM: x86/tsc: Don't sync user-written TSC against startup values
      https://github.com/kvm-x86/linux/commit/bf328e22e472

--
https://github.com/kvm-x86/linux/tree/next