Re: [Qemu-devel] Re: QEMU-KVM and video performance

[Avi Kivity <avi@redhat.com>]

On 04/21/2010 09:50 PM, Gerhard Wiesinger wrote:
>>> I don't think changing VGA window is a problem because there are
>>> 500.000-1Mio changes/s possible.
>>
>> 1MB/s, 500k-1M changes/s.... Coincidence?  Is it taking a page fault
>> or trap on every write?
>
>
> To clarify:
> Memory Performance writing to segmen A000 is about 1MB/st.

That indicates a fault every write (assuming 8-16 bit writes).  If 
you're using 256 color vga and not switching banks, this indicates a bug.

> Calling INT 10 set/get window function with different windows (e.g. 
> toggling between window page 0 and 1) is about 500.000 to 1Mio 
> function calls per second.

That's suprisingly fast. I'd expect 100-200k/sec.

Please run kvm_stat and report output for both tests to confirm.

>
> To get real good VGA performance both parameters should be:
> About >50MB/s for writes to segment A000
> ~500.000 bank switches per second.

First should be doable easily, second is borderline.

> I think this is very easy to distingish:
> 1.) VGA Segment A000 is legacy and should be handled through QEMU and 
> not through KVM (because it is much more faster). Also 16 color modes 
> should be fast enough there.
> 2.) All other flat PCI memory accesses should be handled through KVM 
> (there is a specialized driver loaded for that PCI device in the non 
> legacy OS).
>
> Is that easily possible?

No.  Code can run in either qemu or kvm, not both.  You can switch 
between them based on access statistics (early versions of qemu-kvm did 
that, without the statistics part), but this isn't trivial.

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