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Re: [Qemu-devel] [PATCH v5 09/11] pci bridge dev: change msi property ty

From: Cao jin
Subject: Re: [Qemu-devel] [PATCH v5 09/11] pci bridge dev: change msi property type
Date: Tue, 17 May 2016 15:39:14 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

On 05/15/2016 09:25 PM, Marcel Apfelbaum wrote:
On 05/06/2016 07:20 AM, Cao jin wrote:
 From bit to enum OnOffAuto.

cc: Michael S. Tsirkin <address@hidden>
cc: Markus Armbruster <address@hidden>
cc: Marcel Apfelbaum <address@hidden>
Signed-off-by: Cao jin <address@hidden>

Actually, I am not quite sure this device need this change, RFC.

Well, it already has the 'msi' property, so we may want to make it
standard 'OnOffAuto'.
One problem I can see is the change of semantics. Until now msi=on means
'auto'. From now on
it means 'force msi=on', fail otherwise. If I got this right,  old
machines having msi=on
will failed to start on platforms with msibroken=true, right?

Exactly, and patch 11 indeed has semantics change. According to what we discussed before: "if user want msi=on explicitly on command line, then msi_init`s failure should result in failing to create device", this semantics change seems can`t be avoid.

Maybe we should preserve the semantics for old machines? (this patch
does not actually
affect the semantics, but patch 11/11 should, otherwise why change it to
OnOffAuto, right? )

IMHO, in this case, keep compat maybe a burden on us, and make command-line confusing. See my understanding:

command line format is msi=on/off, and 'on' means auto

we change command line to msi=on/off/auto, and keep compat is meaning 'on' still means auto? If so, why need 'auto'

So, I am thinking, maybe we can help old machine user update their configuration, I mean: when they set msi=on on platforms with msibroken=true, they definitely fail, so we should give a clear hint in error message to help them know what should do to start the machine, like "please use msi=auto and try again"

Yours Sincerely,

Cao jin

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