Re: [Qemu-devel] Re: How to map PCI memory into the VM without trapping

[Cam Macdonell <cam@cs.ualberta.ca>]

On Wed, May 5, 2010 at 5:20 PM, Frank Berreth <frankber@google.com> wrote:
> Hi there,
> this is just an update if you are interested in the outcome. I turns out
> that my initial assumption that there would be page faults/trapping on the
> memory pages was false. The reason the throughput is so low is because the
> memory was mapped non-cached. The VGA driver and the ivshmem driver use
> pci_ioremap_bar which will *always* map the PCI bar non-cached (even the
> resourceX_wc).
> Changing the driver(s) to use ioremap_cache or ioremap_wc speeds up things
> quite a bit. I don't know if VGA framebuffer was always mapped this way --
> it appears on a real system that usually graphics memory is mapped WC.
> Mapping it UC would cause quite a performance degradation. This could be the
> reason for the reported VGA performance drop in another email thread. IMHO,
> since QEMU emulates VGA, this could be mapped WB.
> Thanks,
> Frank.
>

Hi Frank,

Thanks for the note.  I'll make the change to cached and see if that
helps with some of the tests I'm running.

Cheers,
Cam