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

[David Woodhouse]

On Wed, 2023-09-13 at 07:50 -0700, Sean Christopherson wrote:
> On Wed, Sep 13, 2023, David Woodhouse wrote:
> > Userspace used to be able to force a sync by writing zero. You are
> > removing that from the ABI without any explanation about why;
> 
> No, my suggestion did not remove that from the ABI.  A @user_value of '0' would
> still force synchronization.

Ah, OK. Yes, you're right. Thanks.

> It's necessary for "user_set_tsc" to be an accurate name.  The code in v6 yields
> "user_set_tsc_to_non_zero_value".  And I don't think it's just a naming issue,

In another thread, you said that the sync code doesn't differentiate
between userspace initializing the TSC And userspace attempting to
synchronize the TSC. I responded that *I* don't differentiate the two
and couldn't see the difference.

I think we were both wrong. Userspace does *explicitly* synchronize the
TSC by writing zero, and the sync code *does* explicitly handle that,
yes?

And the reason I mention it here is that we could perhaps reasonable
say that userspace *syncing* the TSC like that is not the same as
userspace *setting* the TSC, and that it's OK for user_set_tsc to
remain false? It saves adding another argument to kvm_synchronize_tsc()
making it even more complex for a use case that just doesn't make sense
anyway...

> 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, and it doesn't make much sense
for the user to write 0 to trigger a sync immediately after creating,
because the *kernel* does that anyway.

I don't feel particularly strongly. Having a commit message and code
comments which clearly set out the reasoning for the 1-second slop and
the reasons why we want to *stop* doing it in this case, is the
important part. That had got lost.

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