[Qemu-devel] Kernel mode VGAs?

[Qemu-devel] Kernel mode VGAs?

[Gerhard Wiesinger]

Hello,

Current QEMU-KVM VGA implementation have the following problem with legacy 
OS (e.g. DOS with INT10h calls): Performance is low on accessing A000:0
page and doing bank switching at the 64k page.

Would a kernel mode VGA solve these problems?
How complicated is it?
Is it possible to have only some parts in kernel mode?
Any further ideas or suggestions?

Thnx.

Ciao,
Gerhard

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

Re: [Qemu-devel] Kernel mode VGAs?

[Jan Kiszka]

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On 2012-02-14 08:12, Gerhard Wiesinger wrote:
> Hello,
> 
> Current QEMU-KVM VGA implementation have the following problem with
> legacy OS (e.g. DOS with INT10h calls): Performance is low on accessing
> A000:0
> page and doing bank switching at the 64k page.

Do we already understand the mode and access patterns here? Which VGA
adapter? Cirrus, standard, or any? What is the concrete test case (one
that won't require me digging for MS Dose floppy disks in my basement)?

> 
> Would a kernel mode VGA solve these problems?
> How complicated is it?
> Is it possible to have only some parts in kernel mode?
> Any further ideas or suggestions?

Provided we take heavy exits so far, in-kernel acceleration may reduce
the exit overhead by factor, hmm, maybe 3-4. Better is to avoid exists
completely, i.e. switch the region to RAM mode. But that depends on the
graphic mode, and I'm afraid we have already covered all which can be
mapped like this.

In any case, before discussing solutions, we need to analyze the problem.

Jan


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