Re: [PATCH v6 03/11] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age_gfn

[Sean Christopherson]

On Mon, Aug 19, 2024, Oliver Upton wrote:
> On Fri, Aug 16, 2024 at 07:03:27PM -0600, Yu Zhao wrote:
> > On Fri, Aug 16, 2024 at 6:46 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> 
> [...]
> 
> > > Were you expecting vCPU runtime to improve (more)?  If so, lack of movement could
> > > be due to KVM arm64 taking mmap_lock for read when handling faults:
> > >
> > > https://lore.kernel.org/all/Zr0ZbPQHVNzmvwa6@xxxxxxxxxx
> > 
> > For the above test, I don't think it's mmap_lock
> 
> Yeah, I don't think this is related to the mmap_lock.
> 
> James is likely using hardware that has FEAT_HAFDBS, so vCPUs won't
> fault for an Access flag update.

Huh, didn't know that was a thing on ARM.  Ooh, that lends even more credence to
my assertion that marking folios accessed in handle_access_fault() can go away[*].
I assume hardware-assisted updates means this code in handle_access_fault() will
no longer be hit, as KVM simply won't ever get access faults?  If so, I'll add
that info to the changelog.

	if (kvm_pte_valid(pte))
		kvm_set_pfn_accessed(kvm_pte_to_pfn(pte));

[*] https://lore.kernel.org/all/20240726235234.228822-83-seanjc@xxxxxxxxxx

> Even if he's on a machine w/o it, Access flag faults are handled outside the
> mmap_lock.

Oh, right, they go down handle_access_fault(), not user_mem_abort().

Reviewing late Friday afternoon, never a good idea ;-)