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Re: [Xen-devel] [PATCH 1/4] Add XEN pvSCSI protocol description

On 06/30/2014 01:02 PM, Jan Beulich wrote:
On 27.06.14 at 19:11, <konrad.wilk@xxxxxxxxxx> wrote:
On Fri, Jun 27, 2014 at 04:34:33PM +0200, jgross@xxxxxxxx wrote:
+ * Maximum scatter/gather segments per request.
+ *
+ * Considering balance between allocating at least 16 "vscsiif_request"
+ * structures on one page (4096 bytes) and the number of scatter/gather
+ * elements needed, we decided to use 26 as a magic number.
+ */
+#define VSCSIIF_SG_TABLESIZE           26
+ * based on Linux kernel 2.6.18

This being a bit more .. new, - do these sizes make sense anymore?
Should they be extended a bit? Or have support for using the
old ones (as default) and then negotiate new sizes with the kernel?
(If of course there is a difference?)

As JÃrgen already said (and as you should have noticed yourself) -
this is an interface definition that we can't just change. Negotiation
of larger counts is an option, which is what VSCSIIF_ACT_SCSI_SG_PRESET
is intended for (the implementation of which isn't part of this patchset
afaics, but could be made available on top of it).

I don't think this is the proper way to handle larger SG lists. The
VSCSIIF_ACT_SCSI_SG_PRESET option would just bump the maximum length
up to 31 (currently 26). Or was it thought to be incremental (multiple
presets for one request)? I'd rather add a way to specify SG lists
residing in an own (granted) page (or even multiple pages). This would
allow 512*26 SG entries without having to change the request structure.
The capability to handle this feature could be indicated via xenstore.

Most if not all of your other comments are a little questionable too
in this context - any parts you aren't happy about would really
better be addressed towards the canonical header in the Xen tree.
(I know you had other interface headers diverge too in their Linux
incarnation, but personally I don't think this is an appropriate thing
to do, perhaps apart for pure coding style adjustments.)


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