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Re: [Xen-devel] block ring interface: nr_segments = 0 results in BLKIF_RSP_ERROR
- To: "Chris Youb" <chris_youb@xxxxxxxx>
- From: "Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>
- Date: Thu, 24 Aug 2006 15:00:41 -0700
- Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 24 Aug 2006 15:01:06 -0700
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See dispatch_rw_block_io in blkback.c:
/* Check that number of segments is sane. */
nseg = req->nr_segments;
if (unlikely(nseg == 0) ||
unlikely(nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
DPRINTK("Bad number of segments in request (%d)\n", nseg);
goto fail_response;
}
Why is it important that the backend report that a request for
zero-length i/o is okay?
a.
On 8/24/06, Chris Youb <chris_youb@xxxxxxxx> wrote:
I am currently developing a blkfront.c for a custom OS over Xen 3.0.2-2.
Typical I/O is working, however, I ran into an error while testing a corner
case.
On standard I/O, where { 1 <= nr_segments < BLKIF_MAX_SEGMENTS_PER_REQUEST
} blkif_int()'s bret->status returns BLKIF_RSP_OKAY.
Yet when { nr_segments == 0 } blkif_int's bret->status is non-zero. (Yes I
realize this is an I/O call of zero-length.)
I checked the documentation and section "8.2.2 Block ring interface" states
the following but doesn't exclude 0:
"nr_segments
number of segments for scatter / gather IO described by this request"
1) Is it possible there is a problem w/ my front-end driver (ie does anyone
else see this behaviour)?
2) If this is back-end related, shouldn't it just return BLKIF_RSP_OKAY?
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