linux-kernel - Re: [PATCH 2/3] KVM: x86/mmu: remove unnecessary "bool shared" argument from iterators

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Message-ID: <ZRb31g0PBR588XwK@google.com>
Date:   Fri, 29 Sep 2023 09:14:14 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: x86/mmu: remove unnecessary "bool shared"
 argument from iterators

On Thu, Sep 28, 2023, Maxim Levitsky wrote:
> У чт, 2023-09-28 у 12:29 -0400, Paolo Bonzini пише:
> > The "bool shared" argument is more or less unnecessary in the
> > for_each_*_tdp_mmu_root_yield_safe() macros.  Many users check for
> > the lock before calling it; all of them either call small functions
> > that do the check, or end up calling tdp_mmu_set_spte_atomic() and
> > tdp_mmu_iter_set_spte().  Add a few assertions to make up for the
> > lost check in for_each_*_tdp_mmu_root_yield_safe(), but even this
> > is probably overkill and mostly for documentation reasons.
> 
> Why not to leave the 'kvm_lockdep_assert_mmu_lock_held' but drop the shared
> argument from it?  and then use lockdep_assert_held. If I am not mistaken,
> lockdep_assert_held should assert if the lock is held for read or write.

+1, I don't see any downside to asserting that mmu_lock is held when iterating.

It'll be a redundant assertion 99% of the time, but it's not like performance
matters all that much when running with lockdep enabled.  And I find lockdep
assertions to be wonderful documentation.