Re: [PATCH v2 23/28] KVM: x86/mmu: Allow parallel page faults for the TDP MMU

[Ben Gardon]

On Wed, Feb 3, 2021 at 4:40 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 02/02/21 19:57, Ben Gardon wrote:
> >
> > -     write_lock(&vcpu->kvm->mmu_lock);
> > +
> > +     if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
> > +             read_lock(&vcpu->kvm->mmu_lock);
> > +     else
> > +             write_lock(&vcpu->kvm->mmu_lock);
> > +
>
> I'd like to make this into two helper functions, but I'm not sure about
> the naming:
>
> - kvm_mmu_read_lock_for_root/kvm_mmu_read_unlock_for_root: not precise
> because it's really write-locked for shadow MMU roots
>
> - kvm_mmu_lock_for_root/kvm_mmu_unlock_for_root: not clear that TDP MMU
> operations will need to operate in shared-lock mode
>
> I prefer the first because at least it's the conservative option, but
> I'm open to other opinions and suggestions.
>
> Paolo
>

Of the above two options, I like the second one, though I'd be happy
with either. I agree the first is more conservative, in that it's
clear the MMU lock could be shared. It feels a little misleading,
though to have read in the name of the function but then acquire the
write lock, especially since there's code below that which expects the
write lock. I don't know of a good way to abstract this into a helper
without some comments to make it clear what's going on, but maybe
there's a slightly more open-coded compromise:
if (!kvm_mmu_read_lock_for_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
         write_lock(&vcpu->kvm->mmu_lock);
or
enum kvm_mmu_lock_mode lock_mode =
get_mmu_lock_mode_for_root(vcpu->kvm, vcpu->arch.mmu->root_hpa);
....
kvm_mmu_lock_for_mode(lock_mode);

Not sure if either of those are actually clearer, but the latter
trends in the direction the RCF took, having an enum to capture
read/write and whether or not yo yield in a lock mode parameter.