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

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Message-ID: <56BE20CF.8030708@redhat.com>
Date:	Fri, 12 Feb 2016 19:13:35 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Suravee Suthikulpanit <suravee.suthikulpanit@....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

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?

(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