Re: [Qemu-devel] [PATCH v2 3/7] block/nbd-client: refactor reading reply

[Eric Blake <eblake@redhat.com>]

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On 09/18/2017 10:43 AM, Paolo Bonzini wrote:
> On 18/09/2017 15:59, Vladimir Sementsov-Ogievskiy wrote:
>> Read the whole reply in one place - in nbd_read_reply_entry.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> ---
>>  block/nbd-client.h |  1 +
>>  block/nbd-client.c | 42 ++++++++++++++++++++++++------------------
>>  2 files changed, 25 insertions(+), 18 deletions(-)
>>

> 
> I am not sure this is an improvement.  In principle you could have
> commands that read replies a bit at a time without using a QEMUIOVector.

Right now we don't, but the most likely point where this would be an
issue is the fact that we want to implement structured replies (the
server can send more than one response to a single request from the
client) in order to then implement block status queries (where the
server can send piecemeal information in response to a query, and the
client could very easily want to handle information as it comes in
rather than waiting for the entire server response, especially if the
amount of information returned by the server is not known a priori by
the client, the way the length is known in advance for NBD_CMD_READ, but
instead learned partway through the reply).  I guess the question
becomes a matter of whether we are over-constraining future additions by
making this refactoring, or whether we can still implement block status
queries using a single QEMUIOVector.

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


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