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Re: [Qemu-devel] [RFC] Coalesced MMIO support


From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC] Coalesced MMIO support
Date: Fri, 05 Dec 2008 20:46:50 +0200
User-agent: Thunderbird 2.0.0.18 (X11/20081119)

Anthony Liguori wrote:
The images I need to test this against are not on this computer, so I can't commit until tomorrow when I have access to them again. So I figured that was a good reason to post first as an RFC.

MMIO exits are more expensive in KVM or Xen than in QEMU because they involve, at least, privilege transitions. However, MMIO write operations can be effectively batched if those writes do not have side effects.

Good examples of this include VGA pixel operations when in a planar mode. As it turns out, we can get a nice boost in other areas too. Laurent mentioned a 9.7% performance boost in iperf with the coalesced MMIO changes for the e1000 when he originally posted this work for KVM.


The cirrus blitter could also use this. None of the registers except start/status have any effect except for queuing a bit of data.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.





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