Re: Async page fault delivered while irq are disabled?

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Dec 19, 2019 at 05:15:25PM +0100, Frederic Weisbecker wrote:
> On Thu, Dec 19, 2019 at 07:57:46AM -0800, Sean Christopherson wrote:
> > On Thu, Dec 19, 2019 at 04:28:15PM +0100, Frederic Weisbecker wrote:
> > > Hi,
> > > 
> > > While checking the x86 async page fault code, I can't
> > > find anything that prevents KVM_PV_REASON_PAGE_READY to be injected
> > > while the guest has interrupts disabled. If that page fault happens
> > > to trap in an interrupt disabled section, there may be a deadlock due to the
> > > call to wake_up_process() which locks the rq->lock (among others).
> > > 
> > > Given how long that code is there, I guess such an issue would
> > > have been reported for a while already. But I just would like to
> > > be sure we are checking that.
> > > 
> > > Can someone enlighten me?
> > 
> > The check is triggered from the caller of kvm_async_page_present().
> > 
> > kvm_check_async_pf_completion()
> > |
> > |-> kvm_arch_can_inject_async_page_present()
> >     |
> >     |-> kvm_can_do_async_pf()
> >         |
> >         |-> kvm_x86_ops->interrupt_allowed()
> 
> Ah thanks, I missed that one. And what about
> kvm_async_page_present_sync()? I don't see a similar check
> there.

CONFIG_KVM_ASYNC_PF_SYNC is selected only by s390, it can't be turned on
for x86.

> And one last silly question, what about that line in
> kvm_arch_can_inject_async_page_present:
> 
> 	if (!(vcpu->arch.apf.msr_val & KVM_ASYNC_PF_ENABLED))
> 		return true;
> 
> That looks weird, also it shortcuts the irqs_allowed() check.

I wondered about that code as well :-).  Definitely odd, but it would
require the guest to disable async #PF after an async #PF is queued.  Best
guess is the idea is that it's the guest's problem if it disables async #PF
on the fly.