linux-kernel - Re: [PATCH v5 04/12] Provide special async page fault handler when async PF capability is detected

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Date:	Mon, 23 Aug 2010 11:52:23 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>, kvm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, hpa@...or.com,
	cl@...ux-foundation.org, mtosatti@...hat.com
Subject: Re: [PATCH v5 04/12] Provide special async page fault handler when
 async PF capability is detected

On 08/23/2010 11:48 AM, Avi Kivity wrote:

> Do you need to match cpu here as well? Or is token globally unique?
>
> Perhaps we should make it locally unique to remove a requirement from
> the host to synchronize? I haven't seen how you generate it yet.

If a task goes to sleep on one VCPU, but that VCPU ends
up not being runnable later on, it would be nice to wake
the task up on on a different VCPU.

I do not remember why it is safe to send this wakeup
event as an exception rather than an interrupt...

> The other cpu might be waiting for us to yield. We can fix it later with
> the the pv spinlock infrastructure.
>
> Or, we can avoid the allocation. If at most one apf can be pending (is
> this true?), we can use a per-cpu variable for this dummy entry.

Having a limit of just one APF pending kind of defeats
the point.

At that point, a second one of these faults would put
the VCPU to sleep, which prevents the first task from
running once its pagefault (which started earlier)
completes...

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