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Re: [Xen-devel] [PATCH] patch to buffer write ioreq

On 21 Jun 2006, at 16:11, Li, Xin B wrote:

When we boot win2k guests, just after windows enters standard VGA
graphics mode, it keeps updating a flashing bar to indicate it's in
booting process, but seems this screen updating in win2k guest is
executed as a very high priority task, so win2k can not boot.
The phenomenon is, If in 10ms, about 600 standard VGA MMIO writes can
happen, win2k can boot normally. If less or about 500 writes happen, it
can _not_ boot and just keeps updating the flashing bar, we ever waited
for several hours to see if it can boot, but the guest made no more
progress. I guess there is a threshold for win2k to boot.

That's quite amusing! Of course, even with batching the machine may be slow or loaded enough to cause problems...

I wonder if putting the VGA emulation in Xen itself would make sense? The 'backend protocol' is just a simple linear framebuffer after all: nothing very complicated (much simpler and easier to put in Xen than xenbus/blk/net). VGA MMIO access would then be about as "fast" as a vmenter/vmexit roundtrip. The main downside is that it might limit what tricks we can do with the backend video protocol in future.

 -- Keir

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