Re: [Xen-devel] [PATCH for-4.12] libxl: When restricted, start QEMU paused

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Anthony PERARD writes ("Re: [PATCH for-4.12] libxl: When restricted, start QEMU 
paused"):
> On Wed, Jan 30, 2019 at 03:09:45PM +0000, Ian Jackson wrote:
> >   - libxl connects and hand-checks [xxx???] with QEMU, then sends the
> >     cmd "query-status".
> >   - QEMU prepares and maybe tries to send the response,
> >     while also writing "running" into xenstore.
> >   - libxl sees via xenstore that QEMU is running and disconnects from the
> >     QMP socket before receiving the response from the cmd.
> >   => The QMP socket (monitor) is thereby blocked and will never reply
> >     to commands on new connections.
> > 
> >   This is due to QEMU only responding to one command at a time, and
> >   suspending its monitor (QMP) until the command as been processed and
> >   sent. Disconnecting from the socket doesn't unsuspend the monitor. The
> >   race described here is very likely to happen with QEMU 3.1.50 (during
> >   3.2 development), but can be reproduced with QEMU 3.1.
> > 
> >   [xxx??? So, require, therefore, that when we get the QMP readiness
> >   notification, the qemu state is xenstore.]
> 
> Sorry for this been vague. I'm not 100% sure of what QEMU do and when.
> The listing of QEMU's action are probably not accurate.

Hrm.  Maybe you want to write `roughly' or `I think' or something,
then.  I think the description of the QEMU behaviour is useful even if
not 100% accurate.

> Or maybe the issue here is: s/hand-checks/handshake/, I was trying to
> describe the context with as little words as possible before
> getting to the point where things fails.

Yes, `handshake' makes much more sense.  `Hand-check' would mean
`check by hand'.
    https://en.wiktionary.org/wiki/handshake
    https://en.wiktionary.org/wiki/by_hand

> Is that better to understand the context? How much of this would
> actally be useful for the patch description?

Probably what you wrote originally and I edited, with the xxx's fixed.
Assuming it's actually true...

Thanks,
Ian.

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