Re: [PATCH] x86/hvm: don't expose XENFEAT_hvm_pirqs by default

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jan 10, 2024 at 03:47:12PM +0200, Xenia Ragiadakou wrote:
> 
> 
> On 10/1/24 12:26, Jan Beulich wrote:
> > On 10.01.2024 10:53, Roger Pau Monne wrote:
> > > The HVM pirq feature allows routing interrupts from both physical and emulated
> > > devices over event channels, this was done a performance improvement.  However
> > > its usage is fully undocumented, and the only reference implementation is in
> > > Linux.  It defeats the purpose of local APIC hardware virtualization, because
> > > when using it interrupts avoid the usage of the local APIC altogether.
> > 
> > So without sufficient APIC acceleration, isn't this arranging for degraded
> > performance then? IOW should the new default perhaps be dependent on the
> > degree of APIC acceleration?
> > 
> > > It has also been reported to not work properly with certain devices, at least
> > > when using some AMD GPUs Linux attempts to route interrupts over event
> > > channels, but Xen doesn't correctly detect such routing, which leads to the
> > > hypervisor complaining with:
> > > 
> > > (XEN) d15v0: Unsupported MSI delivery mode 7 for Dom15
> > > 
> > > When MSIs are attempted to be routed over event channels the entry delivery
> > > mode is set to ExtINT, but Xen doesn't detect such routing and attempts to
> > > inject the interrupt following the native MSI path, and the ExtINT delivery
> > > mode is not supported.
> > 
> > Shouldn't this be properly addressed nevertheless? The way it's described
> > it sounds as if MSI wouldn't work at all this way; I can't spot why the
> > issue would only be "with certain devices". Yet that in turn doesn't look
> > to be very likely - pass-through use cases, in particular SR-IOV ones,
> > would certainly have noticed.
> 
> The issue gets triggered when the guest performs save/restore of MSIs,
> because PHYSDEVOP_map_pirq is not implemented for MSIs, and thus, QEMU
> cannot remap the MSI to the event channel once unmapped.

I'm kind of confused by this sentence, PHYSDEVOP_map_pirq does support
MSIs, see xc_physdev_map_pirq_msi() helper in Xen code base.

> So, to fix this issue either would be needed to change QEMU to not unmap
> pirq-emulated MSIs or to implement PHYSDEVOP_map_pirq for MSIs.
> 
> But still, even when no device has been passed-through, scheduling latencies
> (of hundreds of ms), were observed in the guest even when running a simple
> loop application, that disappear once the flag is disabled. We did not have
> the chance to root cause it further.

So XENFEAT_hvm_pirqs is causing such latency issues?  That I certainly
didn't notice.

Regards, Roger.