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

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Date:	Sat, 13 Feb 2016 02:55:03 +0700
From:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
To:	Paolo Bonzini <pbonzini@...hat.com>, <joro@...tes.org>,
	<alex.williamson@...hat.com>, <gleb@...nel.org>
CC:	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<wei@...hat.com>, <sherry.hurwitz@....com>
Subject: Re: [PART1 RFC 0/9] KVM: x86: Introduce SVM AVIC support

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