[Spice-devel] "qxl: add optinal 64bit vram bar" patch significantly slows down Spice

[Hans de Goede]

Hi,

On 04/16/2012 11:39 AM, Gerd Hoffmann wrote:
> On 04/06/12 15:54, Hans de Goede wrote:
>> Hi Gerd,
>>
>> While cherry picking some spice patches into my own qemu-kvm-1.0 branch,
>> so that we can add them to Fedora-17 I noticed a significant slowdown
>> after I was done cherry picking. Investigation has shown your
>> "qxl: add optinal 64bit vram bar" patch to be the culprit.
>>
>> I noticed this using an old Fedora-14 32 bits vm with an xorg-x11-drv-qxl
>> patched to use the async methods.
>>
>> If I scroll through the gnome applications menu with the mouse with
>> plain qemu-kvm-1.0 all is fine, but once I add your:
>> "qxl: add optinal 64bit vram bar" the updating of the display
>> becomes noticably slower, the blue bar highlighting the selected
>> menu entry becomes lagged compared to the mouse cursor.
>
> Avi?  This looks like a memory aliasing issue.
>
> Old state:
>    vram bar, backed by memory
>
> New state:
>    vram bar, 64bit, backed by memory, might not be mapped.
>    vram bar, 32bit, alias for the first part of the 64bit bar.
>

Some clarification:

1) The old state had a 32 bit bar
2) In the new state we are going through the 32 bit bar (my testing was
done with a 32 bit Linux guest), which now is an alias. And since seabios
is not 64 bit bar capable atm, the 64 bit bar likely is not mapped, so we're
(likely) going through a 32 bit alias of an unmapped, memory backed, 64 bit bar.

Regards,

Hans