linux-kernel - Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

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Message-ID: <alpine.DEB.2.00.0912221700230.29381@makko.or.mcafeemobile.com>
Date:	Tue, 22 Dec 2009 17:05:10 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Gregory Haskins <gregory.haskins@...il.com>
cc:	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	kvm@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org,
	"alacrityvm-devel@...ts.sourceforge.net" 
	<alacrityvm-devel@...ts.sourceforge.net>
Subject: Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

On Tue, 22 Dec 2009, Gregory Haskins wrote:

> On 12/22/09 2:39 PM, Davide Libenzi wrote:
> > On Tue, 22 Dec 2009, Gregory Haskins wrote:
> > 
> >> On 12/22/09 1:53 PM, Avi Kivity wrote:
> >>> I asked why the irqfd/ioeventfd mechanisms are insufficient, and you did not reply.
> >>>
> >>
> >> BTW: the ioeventfd issue just fell through the cracks, so sorry about
> >> that.  Note that I have no specific issue with irqfd ever since the
> >> lockless IRQ injection code was added.
> >>
> >> ioeventfd turned out to be suboptimal for me in the fast path for two
> >> reasons:
> >>
> >> 1) the underlying eventfd is called in atomic context.  I had posted
> >> patches to Davide to address that limitation, but I believe he rejected
> >> them on the grounds that they are only relevant to KVM.
> > 
> > I thought we addressed this already, in the few hundreds of email we 
> > exchanged back then :)
> 
> We addressed the race conditions, but not the atomic callbacks.  I can't
> remember exactly what you said, but the effect was "no", so I dropped it. ;)
> 
> This was the thread.
> 
> http://www.archivum.info/linux-kernel@vger.kernel.org/2009-06/08548/Re:_[KVM-RFC_PATCH_1_2]_eventfd:_add_an_explicit_srcu_based_notifier_interface

Didn't that ended up in schedule_work() being just fine, and no need was 
there for pre-emptible callbacks?



> >> 2) it cannot retain the data field passed in the PIO.  I wanted to have
> >> one vector that could tell me what value was written, and this cannot be
> >> expressed in ioeventfd.
> > 
> > Like might have hinted in his reply, couldn't you add data support to the 
> > ioeventfd bits in KVM, instead of leaking them into mainline eventfd?
> > 
> 
> Perhaps, or even easier I could extend xinterface.  Which is what I did ;)
> 
> The problem with the first proposal is that you would no longer actually
> have an eventfd based mechanism...so any code using ioeventfd (like
> Michael Tsirkin's for instance) would break.

At that point, the KVM eventfd can take care of thing so that Michael bits 
do not break.



- Davide


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