Re: [Xen-devel] [PATCH 06/35] libxl: Use ev_qmp for switch_qemu_xen_logdirty

From: Ian Jackson <ian.jackson@citrix.com>
To: Anthony PERARD <anthony.perard@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Wei Liu <wl@xen.org>
Subject: Re: [Xen-devel] [PATCH 06/35] libxl: Use ev_qmp for switch_qemu_xen_logdirty
Date: Thu, 19 Sep 2019 14:03:10 +0100	[thread overview]
Message-ID: <23939.31886.16367.162328@mariner.uk.xensource.com> (raw)
In-Reply-To: <20190919125859.GR1308@perard.uk.xensource.com>

Anthony PERARD writes ("Re: [PATCH 06/35] libxl: Use ev_qmp for switch_qemu_xen_logdirty"):
> On Tue, Sep 17, 2019 at 05:52:24PM +0100, Ian Jackson wrote:
> > I hate to suggest this at this stage, but: maybe the timeout could be
> > incorporated into libxl__ev_qmp ?
> > 
> > I think every libxl__qmp caller is going to need a timeout ?
> 
> Yes, every user of libxl__ev_qmp should have a timeout set up. But
> there are different way to set it up. When we have several command to
> send via QMP should we have a new timeout for every command or set only
> one when sending the first command, and only stopping the timeout when
> the last command's response have been received? In this patch series
> I've choose the second option.
> 
> I can see several cases:
> - One QMP command
>   -> easy, one timeout for it.
> - several commands, related or not.
>   -> one timeout per command? or one timeout which cover all of them?
>      one per commands means that a bad qemu could delay the operation
>      several time longer than if there were a single timeout covering
>      all the commands.
> - other slow operation are done beside a qmp command
>   -> In this case, it might not be practical to have a timeout attach
>      to the qmp command, we are going to need a timeout for the other
>      operations.

Hmmm.  Yes.  You have obviously considered this better than me.q

> I guess we could try to optimise the simpler case when there is only one
> qmp operation, because that's the most common, but allow to send
> commands without having libxl__qmp setting a timeout.

That would be nice.  I think this is a nice-to-have, not a blocker.

Thanks,
Ian.

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