Re: [Xen-devel] [PATCH] linux-2.6.18: improve floppy behavior

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.10.11 at 19:14, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> On 10/11/2011 06:41 AM, Jan Beulich wrote:
>> Timing is significantly different from native both because Xen traps
>> I/O port accesses and since DMA use is not possible (without intrusive
>> changes). Due to the overhead of trapped port accesses, I/O is already
>> slow enough (and Xen doesn't run on very old hardware anyway), so the
>> situation can easily be improved by not enforcing REALLY_SLOW_IO.
>>
>> This doesn't completely address the issue - Xen just cannot guarantee
>> scheduling of a particular vCPU with a maximum latency of about 80us
>> (needed for the default FIFO threshold value of 10). The only complete
>> solution would require making ISA DMA usable on Xen.
> 
> Just out of curiosity, was this in response to a real bug report or
> something?

Indeed, it was, and I was immediately able to repro the problem. Floppy
access, at least on newer kernels (it got reported against 2.6.32.x), did
at best work very sluggishly. 2.6.16.x kernels (on Xen 3.2.x) worked
fine even without the change iirc, and while I recall that I even tried
running the older kernel on newer Xen, I'm not entirely sure anymore
that the problem was really tied to the heavier I/O port access overhead
in newer Xen.

Jan


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