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Date: Tue, 11 Oct 2011 11:56:36 +0200
From: Gleb Natapov <gleb@redhat.com>
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Subject: Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg;
 Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU
 trace backends
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To: Avi Kivity <avi@redhat.com>
Cc: "Richard W.M. Jones" <rjones@redhat.com>, Alexander Graf <agraf@suse.de>, qemu-devel <qemu-devel@nongnu.org>

On Tue, Oct 11, 2011 at 11:49:16AM +0200, Avi Kivity wrote:
> >Whatever we do, the interface will never be as fast as DMA. We will always have to do sanity / permission checks for every IO operation, can batch up only so many IO requests and in QEMU again have to call our callbacks in a loop.
> 
> We can batch per page, which makes the overhead negligible.
> 
Current code batch userspace exit per 1024 bytes IIRC and changing it to
page didn't show significant improvement (also IIRC). But after io data
is copied into the kernel emulator process it byte by byte. Possible
optimization, which I didn't tried, is to check that destination memory is
not mmio and write back the whole buffer if it is the case.

--
			Gleb.