linux-kernel - Re: [PART1 RFC v2 10/10] svm: Manage vcpu load/unload when enable AVIC

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Date:	Mon, 14 Mar 2016 18:58:05 +0700
From:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
To:	Radim Krčmář <rkrcmar@...hat.com>
CC:	<pbonzini@...hat.com>, <joro@...tes.org>, <bp@...en8.de>,
	<gleb@...nel.org>, <alex.williamson@...hat.com>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<wei@...hat.com>, <sherry.hurwitz@....com>
Subject: Re: [PART1 RFC v2 10/10] svm: Manage vcpu load/unload when enable
 AVIC

Hi,

On 03/10/2016 09:01 PM, Radim Krčmář wrote:
> Well, we haven't reached an agreement on is_running yet.  The situation:
> if we don't unset vcpu1.is_running when vcpu1 is scheduled out and vcpu2
> gets scheduled on vcpu1's physical core, then vcpu2 would receive a
> doorbell intended to vcpu1.

That's why, in V2, I added the logic to check if the is_running bit is 
set for the current vcpu (e.g. vcpu1) when unloaded, then restore the 
bit during loading later of if it was set during previous unloaded. This 
way, when we load the new vcpu (e.g. vcpu2), the is_running will be set 
as it was before unloading.

> We'd like to keep is_running set when there is no reason to vmexit, but
> not if a guest can negatively affect other guests.

Not sure how this can affect other guests?

> How does receiving a stray doorbell affect the performance?

As far as I know, the doorbell only affects the CPU during vmrun. Once 
received, it will check the IRR in vAPIC backing page.  So, I think if 
IRR bit is not set, the affect should be rather minimal.

Thanks,
Suravee