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

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Fri, Oct 16, 2020 at 12:44 PM Anthony PERARD
<anthony.perard@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> 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.

I should be able to, yes.

Regards,
Jason