Re: [PATCH 3/3] KVM: arm64: nv: Punt stage-2 recycling to a vCPU request

[Marc Zyngier <maz@kernel.org>]

On Wed, 02 Oct 2024 00:28:18 +0100,
Sean Christopherson <seanjc@google.com> wrote:
> 
> On Tue, Oct 01, 2024, Oliver Upton wrote:
> > Hey,
> > 
> > sidebar: I was a bit confused by the diff for a second, since it looks
> > like your email client lowercased some stuff :)
> 
> Wasn't my mail client, it was PEBKAC.  I copy+pasted a large chunk in Vim because
> I wanted to pull in the changelog (which I had deleted from my response), but then
> I changed my mind, and in doing so I managed to fat-finger something that converted
> everything to lowercase.  And yeah, it confused me too.
> 
> > > >  out:
> > > > +	if (s2_mmu->pending_unmap)
> > > > +		kvm_make_request(kvm_req_nested_s2_unmap, vcpu);
> > > 
> > > If I followed everything correctly, I don't think a request is needed.  the
> > > request will never be cross-vCPU, and each vCPU holds a reference to the MMU, so
> > > the MMU can't be recycled, i.e. pending_unmap is guaranteed to be relevant to the
> > > vCPU's usage of the MMU.  More thoughts below in check_nested_vcpu_requests().
> > 
> > I'm (ab)using the request to prevent the vCPU thread from actually
> > entering the VM without first having done the laundry. We have other
> > examples of strictly per-vCPU tasks that are tracked with a request so
> > this doesn't stick out that much.
> > 
> > Otherwise we'd need an open-coded check in kvm_vcpu_exit_request() to
> > catch a 'dirty' MMU or take a pin on it from the point we check the
> > dirtiness to the point we disable preemption.
> 
> Ewww, because kvm_arch_vcpu_put() puts the nested stage-2 when the vCPU is
> scheduled out.  Mostly out of curiosity, why?  99.9% of the time, the vCPU will
> be scheduled back in.

Because s2 MMU structures are a scarce resource. and other vcpus could
have the opportunity to make use of an unused slot.

> Now that vcpu->scheduled_out is a thing, retaining the nested s2 MMU should be
> quite straightforward.  kvm_arch_vcpu_destroy() would need to put the MMU, but
> that should also be straightforward.

This code long predates scheduled_out, and I don't think this brings
much to the table. If the vcpu comes back quickly, it will find its
toys where it left them. If not, someone else will have borrowed them,
and it will have to pick new ones. It isn't any different from TLBs,
which s2 MMUs model.

	M.

-- 
Without deviation from the norm, progress is not possible.