Re: [Qemu-devel] [PATCH 1/5] nbd: Improve per-export flag handling in server

From: Eric Blake <eblake@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Cc: Kevin Wolf <kwolf@redhat.com>,
	"nbd@other.debian.org" <nbd@other.debian.org>,
	"libguestfs@redhat.com" <libguestfs@redhat.com>,
	"open list:Network Block Dev..." <qemu-block@nongnu.org>,
	Max Reitz <mreitz@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 1/5] nbd: Improve per-export flag handling in server
Date: Fri, 30 Aug 2019 18:10:24 -0500	[thread overview]
Message-ID: <b1483a24-d524-169a-3440-5a9bf4440265@redhat.com> (raw)
In-Reply-To: <92e9cfa2-fbaa-259d-7a04-91187cc809b6@virtuozzo.com>

[-- Attachment #1.1: Type: text/plain, Size: 1286 bytes --]

On 8/30/19 1:00 PM, Vladimir Sementsov-Ogievskiy wrote:
> 23.08.2019 17:37, Eric Blake wrote:
>> When creating a read-only image, we are still advertising support for
>> TRIM and WRITE_ZEROES to the client, even though the client should not
>> be issuing those commands.  But seeing this requires looking across
>> multiple functions:
>>

>> @@ -458,10 +458,13 @@ static int nbd_negotiate_handle_export_name(NBDClient *client,
>>           return -EINVAL;
>>       }
>>
>> -    trace_nbd_negotiate_new_style_size_flags(client->exp->size,
>> -                                             client->exp->nbdflags | myflags);
>> +    myflags = client->exp->nbdflags;
>> +    if (client->structured_reply) {
>> +        myflags |= NBD_FLAG_SEND_DF;
>> +    }
> 
> 
> why we cant do just
> client->exp->nbdflags |= NBD_FLAG_SEND_DF ?

Because myflags is the runtime flags for _this_ client, while
client->exp->nbdflags are the base flags shared by _all_ clients.  If
client A requests structured reply, but client B does not, then we don't
want to advertise DF to client B; but amending client->exp->nbdflags
would have that effect.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]