Re: [External] Re: Re: [RFC] KVM: x86: SVM: don't expose PV_SEND_IPI feature with AVIC

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 16, 2021, 黄科乐 wrote:
> > The recently posted Intel IPI virtualization will accelerate unicast
> > ipi but not broadcast ipis, AMD AVIC accelerates unicast ipi well but
> > accelerates broadcast ipis worse than pv ipis. Could we just handle
> > unicast ipi here?
> 
> Thanks for the explanation! It is true that AVIC does not always perform
> better
> than PV IPI, actually not even swx2apic.
> 
> > So agree with Wanpeng's point, is it possible to separate single IPI and
> > broadcast IPI on a hardware acceleration platform?
> 
> 
> > how about just correcting the logic for xapic:
> 
> > From 13447b221252b64cd85ed1329f7d917afa54efc8 Mon Sep 17 00:00:00 2001
> > From: Jiaqing Zhao <jiaqing.zhao@intel.com>
> > Date: Fri, 9 Apr 2021 13:53:39 +0800
> > Subject: [PATCH 1/2] x86/apic/flat: Add specific send IPI logic
> 
> > Currently, apic_flat.send_IPI() uses default_send_IPI_single(), which
> > is a wrapper of apic->send_IPI_mask(). Since commit aaffcfd1e82d
> > ("KVM: X86: Implement PV IPIs in linux guest"), KVM PV IPI driver will
> > override apic->send_IPI_mask(), and may cause unwated side effects.
> 
> > This patch removes such side effects by creating a specific send_IPI
> > method.
> 
> > Signed-off-by: Jiaqing Zhao <jiaqing.zhao@intel.com>
> 
> Actually, I think this issue is more about how to sort out the relationship
> between AVIC and PV IPI. As far as I understand, currently, no matter
> the option from userspace or the determination made in kernel works
> in some way, but not in the migration scenario. For instance, migration with
> AVIC feature changes can make guests lose the PV IPI feature needlessly.
> Besides, the current patch is not consistent with
> KVM_CAP_ENFORCE_PV_FEATURE_CPUID.
> Paolo's advice about using a new hint shall work well. Currently try
> working on it.

IIUC, you want to have the guest switch between AVIC and PV IPI when the guest
is migrated?  That doesn't require a new hint, it would be just as easy for the
host to manipulate CPUID.KVM_FEATURE_PV_SEND_IPI as it would a new CPUID hint.

The real trick will be getting the guest to be aware of the CPUID and reconfigure
it's APIC setup on the fly.

Or did I misundersetand what you meant by "migration with AVIC feature changes"?