RE: [PATCH v2] x86/hyperv: make vapic support x2apic mode

[Michael Kelley]

From: Roman Kagan <rkagan@xxxxxxxxxxxxx> Sent: Thursday, October 3, 2019 5:53 AM
> >
> > AFAIU you're trying to mirror native_x2apic_icr_write() here but this is
> > different from what hv_apic_icr_write() does
> > (SET_APIC_DEST_FIELD(id)).
> 
> Right.  In xapic mode the ICR2 aka the high 4 bytes of ICR is programmed
> with the destination id in the highest byte; in x2apic mode the whole
> ICR2 is set to the 32bit destination id.
> 
> > Is it actually correct? (I think you've tested this and it is but)
> 
> As I wrote in the commit log, I haven't tested it in the sense that I
> ran a Linux guest in a Hyper-V VM exposing x2apic to the guest, because
> I didn't manage to configure it to do so.  OTOH I did run a Windows
> guest in QEMU/KVM with hv_apic and x2apic enabled and saw it write
> destination ids unshifted to the ICR2 part of ICR, so I assume it's
> correct.
> 
> > Michael, could you please shed some light here?
> 
> Would be appreciated, indeed.
> 

The newest version of Hyper-V provides an x2apic in a guest VM when the
number of vCPUs in the VM is > 240.  This version of Hyper-V is beginning
to be deployed in Azure to enable the M416v2 VM size, but the functionality
is not yet available for the on-premises version of Hyper-V.  However, I can
test this configuration internally with the above patch -- give me a few days.

An additional complication is that when running on Intel processors that offer
vAPIC functionality, the Hyper-V "hints" value does *not* recommend using the
MSR-based APIC accesses.  In this case, memory-mapped access to the x2apic
registers is faster than the synthetic MSRs.  I've already looked at a VM that has
the x2apic, and indeed that is the case, so the above code wouldn't run
anyway.  But I can temporarily code around that for testing purposes and see
if everything works.

Michael