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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 11, 2021 at 12:19:56PM -0400, Peter Xu wrote:
> 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?).

Can't check it in kvm_vcpu_has_events() because that will set
KVM_REQ_UNHALT (which we don't want).

I think KVM_REQ_UNBLOCK will add more lines of code.

> 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.

Hum... IMHO v3 is quite clean already (although i don't object to your
suggestion).

Paolo, what do you think?