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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 17, 2016 at 03:39:14PM +0800, Cao jin wrote:
> 
> 
> 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 <mst@redhat.com>
> >>cc: Markus Armbruster <armbru@redhat.com>
> >>cc: Marcel Apfelbaum <marcel@redhat.com>
> >>Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
> >>---
> >>
> >>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:
> 
> before:
> command line format is msi=on/off, and 'on' means auto
> 
> now:
> 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"

Personally I don't consider this a big issue. I don't think
many people specify msi=on.

> -- 
> Yours Sincerely,
> 
> Cao jin
>