On Mon, 2021-10-25 at 13:43 +0200, Roger Pau Monné wrote:
> It's kind of optional for HVM guests, as it depends on
> XENFEAT_hvm_pirqs, which sadly gets unconditionally set for HVM
> guests, thus dropping any benefits from having hardware assisted APIC
> virtualization or posted interrupts support.

Indeed. After implementing PIRQ support for Xen guests running under
KVM, I spent a "happy" couple of days swearing at it because it
actually *worked* if something would just *unmask* the f***ing MSI, but
the guest inexplicably (to me) didn't do that.

Took me a while to work out that Xen itself is *snooping* on the MSI
table writes even while they are *masked*, to capture the magic MSI
message (with vector==0) which means it's actually a PIRQ# in the
destination ID bits, and then magically unmask the MSI when the guest
binds that PIRQ to an event channel.

I did not enjoy implementing that part.

FWIW the *guest* could potentlaly be smarter here and elect not to use
PIRQs when hardware assisted vAPIC is present. Aren't there some bits
in the CPUID that Xen advertises, which indicate that?