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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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