Re: [PATCH RFC 15/39] KVM: x86/xen: handle PV spinlocks slowpath

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2022-02-10 at 12:17 +0000, Joao Martins wrote:
> On 2/8/22 12:36, David Woodhouse wrote:
> > > +	if (!wait_pending_event(vcpu, sched_poll.nr_ports, ports))
> > > +		wait_event_interruptible_timeout(
> > > +			 vcpu_xen->sched_waitq,
> > > +			 wait_pending_event(vcpu, sched_poll.nr_ports, ports),
> > > +			 sched_poll.timeout ?: KTIME_MAX);
> > 
> > Hm, this doesn't wake on other interrupts, does it? 
> 
> Hmm, I don't think so? This was specifically polling on event channels,
> not sleeping or blocking.
> 
> > I think it should.
> > Shouldn't it basically be like HLT, with an additional wakeup when the
> > listed ports are triggered even when they're masked?
> > 
> 
> I am actually not sure.
> 
> Quickly glancing at the xen source, this hypercall doesn't appear to really
> block the vcpu, but rather just looking if the evtchn ports are pending and
> if a timeout is is specified it sets up a timer. And ofc, wake any evtchn
> pollers. But it doesn't appear to actually block the VCPU. It should be
> IIRC, the functional equivalent of KVM_HC_VAPIC_POLL_IRQ but for event
> channels.
> 

It does block.

https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/common/sched/core.c;hb=RELEASE-4.14.4#l1385

It sets the _VPF_blocked bit in the vCPU's pause_flags, then the
raise_softirq(SCHEDULE_SOFTIRQ) causes it to deschedule this vCPU when
it gets back to the return-to-guest path. It's not Linux, so you don't
see a schedule() call :)

It'll remain blocked until either an (unmasked) event channel or HVM
IRQ is asserted, or one of the masked event channels listed in the poll
list is raised.

Which is why I tried to tie it into the kvm_vcpu_halt() code path when
I updated the patch.


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