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From: Ingo Molnar <mingo@elte.hu>
To: Alexander Graf <agraf@suse.de>
Cc: Sasha Levin <levinsasha928@gmail.com>,
	penberg@kernel.org, kvm@vger.kernel.org, asias.hejun@gmail.com,
	gorcunov@gmail.com, prasadjoshi124@gmail.com
Subject: Re: [PATCH] kvm tools: Add MMIO coalescing support
Date: Sat, 4 Jun 2011 16:46:29 +0200	[thread overview]
Message-ID: <20110604144629.GA22189@elte.hu> (raw)
In-Reply-To: <EBD86BA1-45F6-484D-8FAE-1D82ED0C6FF1@suse.de>


* Alexander Graf <agraf@suse.de> wrote:

> So the simple rule is: don't register a coalesced MMIO region for a 
> region where latency matters. [...]

So my first suspicion is confirmed.

A quick look at Qemu sources shows that lots of drivers are using 
coalesced_mmio without being aware of the latency effects and only 
one seems to make use of qemu_flush_coalesced_mmio_buffer(). Drivers 
like hw/e1000.c sure look latency critical to me.

So i maintain my initial opinion: this is a pretty dangerous 
'optimization' that should be used with extreme care: i can tell it 
you with pretty good authority that latency problems are much more 
easy to introduce than to find and remove ...

Thanks,

	Ingo

  reply	other threads:[~2011-06-04 14:46 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-06-03 19:51 [PATCH] kvm tools: Add MMIO coalescing support Sasha Levin
2011-06-04  9:38 ` Ingo Molnar
2011-06-04 10:14   ` Sasha Levin
2011-06-04 10:17     ` Ingo Molnar
2011-06-04 10:28       ` Sasha Levin
2011-06-04 10:35         ` Ingo Molnar
2011-06-04 10:39           ` Alexander Graf
2011-06-04 10:47             ` Ingo Molnar
2011-06-04 10:54               ` Alexander Graf
2011-06-04 11:27                 ` Ingo Molnar
2011-06-04 11:53                   ` Alexander Graf
2011-06-04 14:46                     ` Ingo Molnar [this message]
2011-06-04 15:22                       ` Alexander Graf
2011-06-04 16:34                         ` Ingo Molnar
2011-06-04 16:50                           ` Sasha Levin

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