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

From: Ben Gardon <bgardon@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	Peter Xu <peterx@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Peter Shier <pshier@google.com>,
	Peter Feiner <pfeiner@google.com>,
	Junaid Shahid <junaids@google.com>,
	Jim Mattson <jmattson@google.com>,
	Yulei Zhang <yulei.kernel@gmail.com>,
	Wanpeng Li <kernellwp@gmail.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: [PATCH v2 23/28] KVM: x86/mmu: Allow parallel page faults for the TDP MMU
Date: Wed, 3 Feb 2021 09:46:30 -0800	[thread overview]
Message-ID: <CANgfPd-ELyPrn5z0N+o8R6Ci=O25XF+EDU-HDGgvVXGV7uF-dQ@mail.gmail.com> (raw)
In-Reply-To: <d2c4ae90-1e60-23ed-4bda-24cf88db04c9@redhat.com>

On Wed, Feb 3, 2021 at 4:40 AM Paolo Bonzini <pbonzini@redhat.com> 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.