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Re: [Qemu-devel] [PATCH v4 4/6] block QMP: Deprecate query-block's "type


From: Luiz Capitulino
Subject: Re: [Qemu-devel] [PATCH v4 4/6] block QMP: Deprecate query-block's "type", drop info block's "type="
Date: Wed, 18 May 2011 10:08:57 -0300

On Wed, 18 May 2011 14:44:11 +0200
Kevin Wolf <address@hidden> wrote:

> Am 16.05.2011 15:04, schrieb Markus Armbruster:
> > query-block's specification documents response member "type" with
> > values "hd", "cdrom", "floppy", "unknown".
> > 
> > Its value is unreliable: a block device used as floppy has type
> > "floppy" if created with if=floppy, but type "hd" if created with
> > if=none.
> > 
> > That's because with if=none, the type is at best a declaration of
> > intent: the drive can be connected to any guest device.  Its type is
> > really the guest device's business.  Reporting it here is wrong.
> > 
> > No known user of QMP uses "type".  It's unlikely that any unknown
> > users exist, because its value is useless unless you know how the
> > block device was created.  But then you also know the true value.
> > 
> > Fixing the broken value risks breaking (hypothetical!) clients that
> > somehow rely on the current behavior.  Not fixing the value risks
> > breaking (hypothetical!) clients that rely on the value to be
> > accurate.  Can't entirely avoid hypothetical lossage.  Change the
> > value to be always "unknown".
> > 
> > This makes "info block" always report "type=unknown".  Pointless.
> > Change it to not report the type.
> > 
> > Signed-off-by: Markus Armbruster <address@hidden>
> 
> Luiz/Anthony, I'm not sure if Markus asked you, but I'm waiting for an
> Ack from one of you here before merging this series. Just in case
> someone wonders why nothing has happened yet.

Sorry for the delay:

ACK

Honestly, I'm a bit reluctant because we don't provide an alternative.
Yes, I understand the value is unreliable, but as far as I understand it
has reliable usages and it's the only way a client can currently learn
about the VM's block devices. However, we don't want new clients to rely
on this field, we're not sure it's used and we're not breaking the
protocol, anyway.



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