linux-kernel - Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow

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Date:   Mon, 5 Jun 2023 20:27:30 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, Jim Mattson <jmattson@...gle.com>
Cc:     Mingwei Zhang <mizhang@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing
 indirect_shadow_pages

On 6/5/23 19:17, Ben Gardon wrote:
>>> -               if (indirect_shadow_pages)
>>> +               if (READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
>> I don't understand the need for READ_ONCE() here. That implies that
>> there is something tricky going on, and I don't think that's the case.
> Agree this doesn't need a READ_ONCE. Just a read is fine.
> kvm_mmu_unprotect_page starts by acquiring the MMU lock, so there's
> not much room to reorder anything anyway.

I agree that "there's not much room to reorder anything", but it's not a 
particularly formal/reassuring statement :) and READ_ONCE/WRITE_ONCE 
have documentation value too.

Or maybe it's because I've become used to the C11 memory model.  Either 
way, I like the idea to use READ_ONCE/WRITE_ONCE more explicitly 
whenever there can be concurrent accesses (which would be data races in 
the C11 model), and I think Linux is moving in that direction too.

Paolo