Re: [PATCH] KVM: x86: async_pf: check earlier if can deliver async pf

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 25, 2024, Nikita Kalyazin wrote:
> On 21/11/2024 21:05, Sean Christopherson wrote:
> > On Thu, Nov 21, 2024, Nikita Kalyazin wrote:
> > > On 19/11/2024 13:24, Sean Christopherson wrote:
> > > > None of this justifies breaking host-side, non-paravirt async page faults.  If a
> > > > vCPU hits a missing page, KVM can schedule out the vCPU and let something else
> > > > run on the pCPU, or enter idle and let the SMT sibling get more cycles, or maybe
> > > > even enter a low enough sleep state to let other cores turbo a wee bit.
> > > > 
> > > > I have no objection to disabling host async page faults, e.g. it's probably a net
> > > > negative for 1:1 vCPU:pCPU pinned setups, but such disabling needs an opt-in from
> > > > userspace.
> > > 
> > > That's a good point, I didn't think about it.  The async work would still
> > > need to execute somewhere in that case (or sleep in GUP until the page is
> > > available).
> > 
> > The "async work" is often an I/O operation, e.g. to pull in the page from disk,
> > or over the network from the source.  The *CPU* doesn't need to actively do
> > anything for those operations.  The I/O is initiated, so the CPU can do something
> > else, or go idle if there's no other work to be done.
> > 
> > > If processing the fault synchronously, the vCPU thread can also sleep in the
> > > same way freeing the pCPU for something else,
> > 
> > If and only if the vCPU can handle a PV async #PF.  E.g. if the guest kernel flat
> > out doesn't support PV async #PF, or the fault happened while the guest was in an
> > incompatible mode, etc.
> > 
> > If KVM doesn't do async #PFs of any kind, the vCPU will spin on the fault until
> > the I/O completes and the page is ready.
> 
> I ran a little experiment to see that by backing guest memory by a file on
> FUSE and delaying response to one of the read operations to emulate a delay
> in fault processing.

...

> In both cases the fault handling code is blocked and the pCPU is free for
> other tasks.  I can't see the vCPU spinning on the IO to get completed if
> the async task isn't created.  I tried that with and without async PF
> enabled by the guest (MSR_KVM_ASYNC_PF_EN).
> 
> What am I missing?

Ah, I was wrong about the vCPU spinning.

The goal is specifically to schedule() from KVM context, i.e. from kvm_vcpu_block(),
so that if a virtual interrupt arrives for the guest, KVM can wake the vCPU and
deliver the IRQ, e.g. to reduce latency for interrupt delivery, and possible even
to let the guest schedule in a different task if the IRQ is the guest's tick.

Letting mm/ or fs/ do schedule() means the only wake event even for the vCPU task
is the completion of the I/O (or whatever the fault is waiting on).