linux-kernel - Re: [PATCH 09/22] KVM: x86/mmu: Try "unprotect for retry" iff there are indirect SPs

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Message-ID: <CABgObfbyJo2uYYkTTYdrrYQcB6XgB2+PhmfqwKrQ-g7D5UPr5A@mail.gmail.com>
Date: Thu, 15 Aug 2024 18:48:36 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Gonda <pgonda@...gle.com>, Michael Roth <michael.roth@....com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Ackerly Tng <ackerleytng@...gle.com>
Subject: Re: [PATCH 09/22] KVM: x86/mmu: Try "unprotect for retry" iff there
 are indirect SPs

On Thu, Aug 15, 2024 at 4:09 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > (This is preexisting in reexecute_instruction() and goes away in patch 18, if
> > I'm pre-reading that part of the series correctly).
> >
> > Bonus points for opportunistically adding a READ_ONCE() here and in
> > kvm_mmu_track_write().
>
> Hmm, right, this one should have a READ_ONCE(), but I don't see any reason to
> add one in kvm_mmu_track_write().  If the compiler was crazy and generate multiple
> loads between the smp_mb() and write_lock(), _and_ the value transitioned from
> 1->0, reading '0' on the second go is totally fine because it means the last
> shadow page was zapped.  Amusingly, it'd actually be "better" in that it would
> avoid unnecessary taking mmu_lock.

Your call, but I have started leaning towards always using
READ_ONCE(), similar to all atomic_t accesses are done with
atomic_read(); that is, just as much as a marker for cross-thread
lock-free accesses, in addition to limiting the compiler's
optimizations.

tools/memory-model/Documentation/access-marking.txt also suggests
using READ_ONCE() and WRITE_ONCE() always except in special cases.
They are also more friendly to KCSAN (though I have never used it).

This of course has the issue of being yet another unfinished transition.

> Obviously the READ_ONCE() would be harmless, but IMO it would be more confusing
> than helpful, e.g. would beg the question of why kvm_vcpu_exit_request() doesn't
> wrap vcpu->mode with READ_ONCE().  Heh, though arguably vcpu->mode should be
> wrapped with READ_ONCE() since it's a helper and could be called multiple times
> without any code in between that would guarantee a reload.

Indeed, who said I wouldn't change that one as well? :)

Paolo