Re: [PART1 RFC 0/9] KVM: x86: Introduce SVM AVIC support

[Suravee Suthikulpanit]

Hi,

On 2/13/16 01:13, Paolo Bonzini wrote:
> I have a few questions about AVIC.
>
> On 12/02/2016 14:59, Suravee Suthikulpanit wrote:
> > CURRENT UNSUPPORT USE-CASES
> > ===========================
> >      - Nested VM
> >      - VM Migration
>
> I'm interested in what you mean with "VM migration".  I've noticed that,
> because x2APIC is disabled by default, it's possible that a VM on a
> non-AVIC host will fail when moved to an AVIC host.  I would prefer if
> you kept x2APIC working through VMEXITs.  Getting the full performance
> benefit would require disabling x2APIC of course (or waiting for
> improved support in the processor), but at least there would be no surprise.
>
> Is it just this, or is there anything else?

For VM Migration, I meant the process of taking a snapshot of the VM and 
later on loading it back to resume where it left off, and not necessary 
to a different type of system. This would require saving off the VAPIC 
backing page of each vCPU along with the rest of per-VM AVIC data 
structures. IIUC, this might require changes in the QEmu as well.

Moving VM non-AVIC host to AVIC host, and vice versa, would be tricky.

I also see you points about supporting x2APIC. Let me look more into it, 
and then get back to you on this.

> (I'm quite surprised that x2APIC is not supported.  MSRs are faster than
> memory accesses due to the cost of TLB misses, which can be hefty for VMs).
>
> I have a few other doubts about the architecture that hopefully you (or
> Wei Huang too) can clarify before I can review this patch meaningfully.
>   I have replied to other patches with these questions.
>
> Paolo

I'll go through those and get back to you separately.

Regards,

Suravee