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

Gerhard Wiesinger
2010-05-09T19:37:35Z

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

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