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

[Gerhard Wiesinger <lists@wiesinger.com>]

On Thu, 22 Apr 2010, Avi Kivity wrote:

> On 04/22/2010 09:04 AM, Gerhard Wiesinger wrote:
>> On Wed, 21 Apr 2010, Avi Kivity wrote:
>> 
>>> 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.
>>> 
>> 
>> Yes, 256 color VGA and no bank switches involved.
>> 
>>>> 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.
>>> 
>> 
>> Sorry, I mixed up the numbers:
>> 1.) QEMU-KVM: ~111k
>> 2.) QEMU only: 500k-1Mio
>> 
>>> Please run kvm_stat and report output for both tests to confirm.
>>> 
>> 
>> See below. 2nd column is per second statistic when running the test.
>
> efer_reload                  0       0
> exits                 18470836  554582
> fpu_reload             2147833    3469
> halt_exits                2083       0
> halt_wakeup               2047       0
> host_state_reload      2148186    3470
> hypercalls                   0       0
> insn_emulation         7688203  554244
>
> This indicates that kvm is emulating instead of direct mapping.  That's 
> probably a bug.  If you fix it, performance will increase dramatically.

Where can I start here?
Any ideas how to?

One of my ideas: Move hw/vga.c functions
vga_mem_readb
vga_mem_readw
vga_mem_readl
vga_mem_writeb
vga_mem_writew
vga_mem_writel
to KVM to avoid switching from KVM to QEMU (I can write C code even 
kernel but I'm not comfortable with KVM). Howto?

>>>> 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.
>> 
>> Hmmm. Ok, 2 different opinions about the memory write performance:
>> Easily or not possible?
>
> Switching between tcg and kvm is hard, but not needed.  For 256 color modes, 
> direct map is possible and should yield good performance.  Bank switching can 
> be improved perhaps 3x, but will never be fast.

Where can I start for KVM performance for the bank switching (256 color 
mode)? (e.g. BIOS writes to VGA window I/O port to switch the bank)
Any ideas how to improve (architecture for the change)?

Thnx and sorry for long delay, was busy.

Ciao,
Gerhard

--
http://www.wiesinger.com/