Re: [Qemu-devel] [PATCH 0/2] ipmi: Allow UUID to be set for a BMC

From: Corey Minyard <minyard@acm.org>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-devel@nongnu.org, "Cédric Le Goater" <clg@kaod.org>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Fam Zheng" <famz@redhat.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 0/2] ipmi: Allow UUID to be set for a BMC
Date: Fri, 9 Nov 2018 07:33:03 -0600	[thread overview]
Message-ID: <8a03cead-db46-f4fb-1e57-30bec0802bc8@acm.org> (raw)
In-Reply-To: <20181108232211.GB26872@umbus.fritz.box>

On 11/8/18 5:22 PM, David Gibson wrote:
> On Thu, Nov 08, 2018 at 08:19:42AM -0600, minyard@acm.org wrote:
>> The code was using the qemu UUID for the BMC.  But that's really
>> not a good method.  In general, you don't want the GUID to change
>> when you migrate, and you want the GUID to be the same between
>> invocations of qemu (if you have a GUID).
> Hrm.  Generally the qemu UUID should remain the same across a
> migration too, and I think that will be the case if using libvirt.
> Maybe not if running qemu by hand and not specifying the uuid on the
> command line.
>
> I don't really have an objection to allowing the BMC's id to be
> explicitly controlled, but the rationale above seems a bit
> disingenuous.

I'm not sure about migration.  I suppose it could be migrated, but I
would consider the BMC part of the hardware that needs to be the
same on both sides.  It's a fuzzy line, I suppose.  The qemu UUID
is migrated, so I suppose that's not an issue.

Controlling it explicitly is important for some testing I do, and might
be for other people at some point in time, if you are trying to
emulate something specific.  And when re-invoking qemu, you
might want to keep it the same to avoid confusing software.

The big question for you is the lack of a GUID if it's not supplied.
With this change, the get GUID command will return a "command
not supported" error if the GUID is not supplied.

Thanks,

-corey

>> Plus, if you have multiple BMCs, they need to have different
>> GUIDs or the host code cannot tell them apart.  I'm not sure
>> anyone really uses multiple BMCs, but I do a lot of testing
>> with that scenario.
>>
>> This change lets the user set the GUID on the command line, and
>> if the GUID is not set return an error for the GUID fetch command.
>> This maps better to how IPMI should work.
>>
>> This change relies on the UUID being set to all zeros to know that
>> it is not set.  This is not optimal, perhaps, but an all zero
>> UUID isn't valid (it's the Nil UUID), so it should be ok.