Re: [Xen-devel] [PATCH 2/3] x86/nvmx: clarify and fix usage of nvmx_update_apicv

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Roger Pau Monné <roger.pau@citrix.com>
> Sent: Tuesday, March 24, 2020 8:19 PM
> 
> On Tue, Mar 24, 2020 at 11:33:00AM +0000, Tian, Kevin wrote:
> > > From: Roger Pau Monné <roger.pau@citrix.com>
> > > Sent: Tuesday, March 24, 2020 7:23 PM
> > >
> > > On Tue, Mar 24, 2020 at 10:11:15AM +0000, Tian, Kevin wrote:
> > > > > From: Roger Pau Monné <roger.pau@citrix.com>
> > > > > Sent: Tuesday, March 24, 2020 5:51 PM
> > > > >
> > > > > On Tue, Mar 24, 2020 at 06:03:28AM +0000, Tian, Kevin wrote:
> > > > > > > From: Roger Pau Monne <roger.pau@citrix.com>
> > > > > > > Sent: Saturday, March 21, 2020 3:08 AM
> > > > > > >
> > > > > > > The current usage of nvmx_update_apicv is not clear: it is deeply
> > > > > > > intertwined with the Ack interrupt on exit VMEXIT control.
> > > > > > >
> > > > > > > The code in nvmx_update_apicv should update the SVI (in service
> > > > > interrupt)
> > > > > > > field of the guest interrupt status only when the Ack interrupt on
> > > > > > > exit is set, as it is used to record that the interrupt being
> > > > > > > serviced is signaled in a vmcs field, and hence hasn't been injected
> > > > > > > as on native. It's important to record the current in service
> > > > > > > interrupt on the guest interrupt status field, or else further
> > > > > > > interrupts won't respect the priority of the in service one.
> > > > > > >
> > > > > > > While clarifying the usage make sure that the SVI is only updated
> > > when
> > > > > > > Ack on exit is set and the nested vmcs interrupt info field is valid.
> Or
> > > > > > > else a guest not using the Ack on exit feature would loose
> interrupts
> > > as
> > > > > > > they would be signaled as being in service on the guest interrupt
> > > > > > > status field but won't actually be recorded on the interrupt info
> vmcs
> > > > > > > field, neither injected in any other way.
> > > > > >
> > > > > > It is insufficient. You also need to update RVI to enable virtual
> injection
> > > > > > when Ack on exit is cleared.
> > > > >
> > > > > But RVI should be updated in vmx_intr_assist in that case, since
> > > > > nvmx_intr_intercept shouldn't intercept the interrupt, as it should be
> > > > > handled normally.
> > > >
> > > > As we discussed before, vmx_intr_assist is invoked before
> > > nvmx_switch_guest.
> > > >
> > > > It is incorrectly to update RVI at that time since it might be still vmcs02
> > > being
> > > > active (if no pending softirq to make it invoked again).
> > > >
> > > > Also nvmx_intr_intercept does intercept Ack-on-exit=0 case:
> > > >
> > > >         if ( intack.source == hvm_intsrc_pic ||
> > > >                  intack.source == hvm_intsrc_lapic )
> 
> I've realized that nvmx_intr_intercept will return 1 for interrupts
> originating from the lapic or the pic, while nvmx_update_apicv will
> only update GUEST_INTR_STATUS for interrupts originating from the
> lapic. Is this correct?

Isn't apicv for virtual lapic instead of virtual pic?

> 
> Shouldn't both be in sync and handle the same interrupt sources?
> 

But I do realize one potential issue about 67f9d0b9, which may break
vpic delivery when ack-on-exit is off. We should always use interrupt
window to handle that situation for vpic. Sorry I didn't catch it when
proposing that change...

Thanks
Kevin