Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Peter Xu <peterx@redhat.com>]

On Tue, May 11, 2021 at 11:51:57AM -0300, Marcelo Tosatti wrote:
> On Tue, May 11, 2021 at 10:39:11AM -0400, Peter Xu wrote:
> > On Fri, May 07, 2021 at 07:08:31PM -0300, Marcelo Tosatti wrote:
> > > > Wondering whether we should add a pi_test_on() check in kvm_vcpu_has_events()
> > > > somehow, so that even without customized ->vcpu_check_block we should be able
> > > > to break the block loop (as kvm_arch_vcpu_runnable will return true properly)?
> > > 
> > > static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> > > {
> > >         int ret = -EINTR;
> > >         int idx = srcu_read_lock(&vcpu->kvm->srcu);
> > > 
> > >         if (kvm_arch_vcpu_runnable(vcpu)) {
> > >                 kvm_make_request(KVM_REQ_UNHALT, vcpu); <---
> > >                 goto out;
> > >         }
> > > 
> > > Don't want to unhalt the vcpu.
> > 
> > Could you elaborate?  It's not obvious to me why we can't do that if
> > pi_test_on() returns true..  we have pending post interrupts anyways, so
> > shouldn't we stop halting?  Thanks!
> 
> pi_test_on() only returns true when an interrupt is signalled by the
> device. But the sequence of events is:
> 
> 
> 1. pCPU idles without notification vector configured to wakeup vector.
> 
> 2. PCI device is hotplugged, assigned device count increases from 0 to 1.
> 
> <arbitrary amount of time>
> 
> 3. device generates interrupt, sets ON bit to true in the posted
> interrupt descriptor.
> 
> We want to exit kvm_vcpu_block after 2, but before 3 (where ON bit
> is not set).

Ah yes.. thanks.

Besides the current approach, I'm thinking maybe it'll be cleaner/less LOC to
define a KVM_REQ_UNBLOCK to replace the pre_block hook (in x86's kvm_host.h):

#define KVM_REQ_UNBLOCK			KVM_ARCH_REQ(31)

We can set it in vmx_pi_start_assignment(), then check+clear it in
kvm_vcpu_has_events() (or make it a bool in kvm_vcpu struct?).

The thing is current vmx_vcpu_check_block() is mostly a sanity check and
copy-paste of the pi checks on a few items, so maybe cleaner to use
KVM_REQ_UNBLOCK, as it might be reused in the future for re-evaluating of
pre-block for similar purpose?

No strong opinion, though.

-- 
Peter Xu