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

["Jan Beulich" <JBeulich@xxxxxxxx>]

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.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -146,7 +146,9 @@
 #define FLOPPY_SANITY_CHECK
 #undef  FLOPPY_SILENT_DCL_CLEAR
 
+#ifndef CONFIG_XEN
 #define REALLY_SLOW_IO
+#endif
 
 #define DEBUGT 2
 #define DCL_DEBUG              /* debug disk change line */




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