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SubjectRe: [PATCH v2] x86/hyperv: make vapic support x2apic mode
On Fri, Oct 04, 2019 at 03:01:51AM +0000, Michael Kelley wrote:
> From: Roman Kagan <> 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 guess you mean "using regular x2apic MSRs compared to the synthetic
MSRs". Indeed they do essentially the same thing, and there's no reason
for one set of MSRs to be significantly faster than the other. However,
hv_apic_eoi_write makes use of "apic assists" aka lazy EOI which is
certainly a win, and I'm not sure if it works without hv_apic.

> 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.


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