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Re: [Qemu-block] [PATCH] doc: Permit BLOCK_STATUS reply to extend beyond

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH] doc: Permit BLOCK_STATUS reply to extend beyond request
Date: Thu, 16 Aug 2018 14:26:46 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 08/15/2018 04:23 AM, Vladimir Sementsov-Ogievskiy wrote:

Finally, Qemu 3.0 released, and it is incompatible with current NBD protocol. Please, commit this patch.

Indeed, qemu 3.0 with the qemu:dirty-bitmap:NAME context does exceed the final length when REQ_ONE is not in force.

As I recall, there was no complaint about the concept, only the wording; so I think this updated wording satisfies everyone:

   If the client used the `NBD_CMD_FLAG_REQ_ONE` flag in the request,
-  then every reply chunk MUST NOT contain more than one descriptor.
+  then every reply chunk MUST contain exactly one descriptor, and that
+  descriptor MUST NOT exceed the *length* of the original request.  If
+  the client did not use the flag, and the server replies with N
+  extents, then the sum of the *length* fields of the first N-1
+  extents (if any) MUST be less than the requested length, while the
+  *length* of the final extent MAY result in a sum larger than the
+  original requested length, if the server has that information anyway
+  as a side effect of reporting the status of the requested region.

I've gone ahead and pushed this.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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