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Re: [Qemu-devel] qvm86, kqemu and video speed

From: Struan Bartlett
Subject: Re: [Qemu-devel] qvm86, kqemu and video speed
Date: Wed, 13 Apr 2005 10:06:08 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.3) Gecko/20041007 Debian/1.7.3-5

Fabrice Bellard wrote:

Alex Beregszaszi wrote:

Struan Bartlett wrote:

I understand qvm86 and kqemu provide some virtualisation of the host machine, including allowing the guest some direct memory access. Is it conceivable for these modules to be extended to allow the guest machine to directly write to host video memory, or else to a host memory buffer that is copied into the Qemu window?

I'm working on such a Direct Host Graphics custom "videocard".

You can do that, but there is also a lot of optimisation opportunities in the existing Cirrus driver. My feeling is that using a driver for a virtual card will add only marginal gains (except in 3d) for a bigger amount of work (you need specific drivers in the guest OSes).

For example, in the Cirrus driver, it could be possible for the virtual CPU to access the virtual frame buffer (currently a callback is always used). A specific virtual CPU support is needed to change dynamically the type of a physical memory mapping - that's why I did not implement it when I enhanced the Cirrus driver. It will be even more critical soon with a more optimized version of kqemu.

Thanks Fabrice for Qemu and for your input to this question. What you describe sounds like the sort of thing I was imagining. Are you suggesting your next version of kqemu will provide the "specific virtual CPU support" needed to do this? If so, please keep us posted.

Moreover, it could be possible to suppress one memcpy from the virtual frame buffer to the SDL/X11 frame buffer, and another memcpy if full screen mode is used (in this case, the virtual CPU accesses directly the host frame buffer).

Finally, the Cirrus bitblt operations could be redirected to the corresponding X11 DGA operations in full screen mode.

Or, do you think, as James Mastros suggested, the corresponding SDL operations in non-full-screen mode?


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