Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow_pages

[Paolo Bonzini]

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