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

From: Avi Kivity
Subject: Re: [Qemu-devel] Re: QEMU-KVM and video performance
Date: Wed, 21 Apr 2010 23:56:57 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

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.

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