From: Anthony PERARD via <qemu-devel@nongnu.org>
To: Jason Andryuk <jandryuk@gmail.com>
Cc: QEMU <qemu-devel@nongnu.org>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
xen-devel <xen-devel@lists.xenproject.org>,
"Paul Durrant" <paul@xen.org>,
"Stefano Stabellini" <sstabellini@kernel.org>,
"Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
"Marcel Apfelbaum" <marcel.apfelbaum@gmail.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Richard Henderson" <rth@twiddle.net>,
"Eduardo Habkost" <ehabkost@redhat.com>
Subject: Re: [PATCH] hw/xen: Set suppress-vmdesc for Xen machines
Date: Fri, 16 Oct 2020 17:44:28 +0100 [thread overview]
Message-ID: <20201016164428.GC3105841@perard.uk.xensource.com> (raw)
In-Reply-To: <CAKf6xpssB-FGwiEhLqV8OFjBGuP4LKYh+9Pj_Bj7p5U2CJSw=g@mail.gmail.com>
On Fri, Oct 16, 2020 at 12:01:47PM -0400, Jason Andryuk wrote:
> On Fri, Oct 16, 2020 at 11:38 AM Anthony PERARD
> <anthony.perard@citrix.com> wrote:
> >
> > On Tue, Oct 13, 2020 at 03:05:06PM -0400, Jason Andryuk wrote:
> > > xen-save-devices-state doesn't currently generate a vmdesc, so restore
> > > always triggers "Expected vmdescription section, but got 0". This is
> > > not a problem when restore comes from a file. However, when QEMU runs
> > > in a linux stubdom and comes over a console, EOF is not received. This
> > > causes a delay restoring - though it does restore.
> > >
> > > Setting suppress-vmdesc skips looking for the vmdesc during restore and
> > > avoids the wait.
> >
> > suppress-vmdesc is only used during restore, right? So starting a guest
> > without it, saving the guest and restoring the guest with
> > suppress-vmdesc=on added will work as intended? (I'm checking that migration
> > across update of QEMU will work.)
>
> vmdesc is a json description of the migration stream that comes after
> the QEMU migration stream. For our purposes, <migration
> stream><vmdesc json blob>. Normal QEMU savevm will generate it,
> unless suppress-vmdesc is set. QEMU restore will read it because:
> "Try to read in the VMDESC section as well, so that dumping tools that
> intercept our migration stream have the chance to see it."
>
> Xen save does not go through savevm, but instead
> xen-save-devices-state, which is a subset of the QEMU savevm. It
> skips RAM since that is read out through Xen interfaces. Xen uses
> xen-load-devices-state to restore device state. That goes through the
> common qemu_loadvm_state which tries to read the vmdesc stream.
>
> For Xen, yes, suppress-vmdesc only matters for the restore case, and
> it suppresses the attempt to read the vmdesc. I think every Xen
> restore currently has "Expected vmdescription section, but got -1" in
> the -dm.log since the vmdesc is missing. I have not tested restoring
> across this change, but since it just controls reading and discarding
> the vmdesc stream, I don't think it will break migration across
> update.
Thanks for the explanation.
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Do you think you could send a patch for libxl as well? Since libxl in
some cases may use the "pc machine instead of "xenfv". I can send the
patch otherwise.
Cheers,
--
Anthony PERARD
next prev parent reply other threads:[~2020-10-16 16:46 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-13 19:05 [PATCH] hw/xen: Set suppress-vmdesc for Xen machines Jason Andryuk
2020-10-16 15:37 ` Anthony PERARD via
2020-10-16 16:01 ` Jason Andryuk
2020-10-16 16:44 ` Anthony PERARD via [this message]
2020-10-16 17:02 ` Jason Andryuk
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