Re: [PATCH] hw/xen: Set suppress-vmdesc for Xen machines

From: Jason Andryuk <jandryuk@gmail.com>
To: Anthony PERARD <anthony.perard@citrix.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Eduardo Habkost" <ehabkost@redhat.com>,
	"Paul Durrant" <paul@xen.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>,
	QEMU <qemu-devel@nongnu.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	"Richard Henderson" <rth@twiddle.net>
Subject: Re: [PATCH] hw/xen: Set suppress-vmdesc for Xen machines
Date: Fri, 16 Oct 2020 12:01:47 -0400	[thread overview]
Message-ID: <CAKf6xpssB-FGwiEhLqV8OFjBGuP4LKYh+9Pj_Bj7p5U2CJSw=g@mail.gmail.com> (raw)
In-Reply-To: <20201016153708.GB3105841@perard.uk.xensource.com>

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.

Regards,
Jason