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

[Avi Kivity <avi@redhat.com>]

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.