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Re: [Xen-devel] floppy not working in Linux

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] floppy not working in Linux
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Thu, 4 Mar 2010 13:28:49 -0500
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On Thu, Mar 04, 2010 at 02:43:51PM +0000, Jan Beulich wrote:
> We had a report that the floppy driver isn't working with current Xen.
> While certainly not a major concern to most of us, it still seems odd that
> this simple a piece of hard-/software wouldn't work under Xen, especially
> as it is known that it had been working a couple of years back.

The USB one works, thought there the DMA Debug interface throws a nice
message about doing DMA from a stack - hadn't looked in details for
in this.
> Trying to debug the issue has surfaced quite a perhaps strange mix of
> observations:
> - On an old (32-bit) box of mine I found that the driver would work
> on Xen 3.1, while the same kernel running on top of 3.2.x would not
> work (nor would a reasonably current kernel on a reasonably current
> Xen).
> - On both this old and not too old a 64-bit box I found that removing
> the REALLY_SLOW_IO definition at the top of floppy.c would reduce
> the amount of errors encountered quite significantly. Specifically, I
> have never seen a mount fail with that change in place (but there
> are occasional errors logged by the driver).
> - Generally, the problems start with unexpected interrupts or, less
> frequently, "-- FDC reply error" messages.
> While the driver certainly didn't change much over the last couple
> of years, the effect of the adjustment above suggests that the
> driver itself may have problems which are just unnoticeable when
> run on bare hardware.
> Given the timing requirements to service the FDC (a maximum of
> about 120us when using the FIFO optimally) I wonder whether it
> is reasonable to expect this to work reliably at all, especially
> when guest domains are consuming non-neglectable amounts of
> CPU time.
> Jan
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