Re: [Qemu-devel] [PATCH] migration: incoming postcopy advise sanity checks

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Vladimir Sementsov-Ogievskiy (vsementsov@virtuozzo.com) wrote:
> 06.02.2018 11:43, Greg Kurz wrote:
> > On Tue, 6 Feb 2018 10:49:47 +0300
> > Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> wrote:
> > 
> > > 06.02.2018 10:26, Greg Kurz wrote:
> > > > If postcopy-ram was set on the source but not on the destination,
> > > > migration doesn't occur, the destination prints an error and boots
> > > > the guest:
> > > > 
> > > > qemu-system-ppc64: Expected vmdescription section, but got 0
> > > > 
> > > > We end up with two running instances.
> > > > 
> > > > This behaviour was introduced in 2.11 by commit 58110f0acb1a "migration:
> > > > split common postcopy out of ram postcopy" to prepare ground for the
> > > > upcoming dirty bitmap postcopy support. It adds a new case where the
> > > > source may send an empty postcopy advise because dirty bitmap doesn't
> > > > need to check page sizes like RAM postcopy does.
> > > > 
> > > > If the source has enabled postcopy-ram, then it sends an advise with
> > > > the page size values. If the destination hasn't enabled postcopy-ram,
> > > > then loadvm_postcopy_handle_advise() leaves the page size values on
> > > > the stream and returns. This confuses qemu_loadvm_state() later on
> > > > and causes the destination to start execution.
> > > > 
> > > > As discussed several times, postcopy-ram should be enabled both sides
> > > > to be functional. This patch changes the destination to perform some
> > > > extra checks on the advise length to ensure this is the case. Otherwise
> > > > an error is returned and migration is aborted.
> > > > 
> > > > Reported-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
> > > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > > ---
> > > >    migration/savevm.c |   18 +++++++++++++++---
> > > >    1 file changed, 15 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/migration/savevm.c b/migration/savevm.c
> > > > index b7908f62be3c..1c516fcbb8d7 100644
> > > > --- a/migration/savevm.c
> > > > +++ b/migration/savevm.c
> > > > @@ -1376,7 +1376,8 @@ static int qemu_loadvm_state_main(QEMUFile *f, MigrationIncomingState *mis);
> > > >     * *might* happen - it might be skipped if precopy transferred everything
> > > >     * quickly.
> > > >     */
> > > > -static int loadvm_postcopy_handle_advise(MigrationIncomingState *mis)
> > > > +static int loadvm_postcopy_handle_advise(MigrationIncomingState *mis,
> > > > +                                         uint16_t len)
> > > >    {
> > > >        PostcopyState ps = postcopy_state_set(POSTCOPY_INCOMING_ADVISE);
> > > >        uint64_t remote_pagesize_summary, local_pagesize_summary, remote_tps;
> > > > @@ -1387,8 +1388,19 @@ static int loadvm_postcopy_handle_advise(MigrationIncomingState *mis)
> > > >            return -1;
> > > >        }
> > > > -    if (!migrate_postcopy_ram()) {
> > > > +    switch (len) {
> > > > +    case 0:
> > > > +        /* The source hasn't enabled postcopy-ram. Nothing to do. */
> > > should we error-out here if (migrate_postcopy_ram()) ?
> > > 
> > I was also thinking so at first, but if the source hasn't enabled postcopy-ram,
> > then RAM postcopy won't happen. Not sure why we should error out...
> 
> if user enables dirty-bitmaps postcopy on source, and enables postcopy-ram
> on target,
> we will be here, with migrate_postcopy_ram()=true. and it should be error.

In that case it's probably worth it.

> > 
> > > >            return 0;
> > > > +    case 8 + 8:
> > > > +        if (!migrate_postcopy_ram()) {
> > > > +            error_report("RAM postcopy is disabled");

Can you make that a little bit more descriptive please:
    error_report("RAM postcopy is disabled but have 16 byte advise");
should do.

Thanks,

Dave

> > > > +            return -EINVAL;
> > > > +        }
> > > > +        break;
> > > > +    default:
> > > > +        error_report("CMD_POSTCOPY_ADVISE invalid length (%d)", len);
> > > > +        return -EINVAL;
> > > >        }
> > > >        if (!postcopy_ram_supported_by_host(mis)) {
> > > > @@ -1807,7 +1819,7 @@ static int loadvm_process_command(QEMUFile *f)
> > > >            return loadvm_handle_cmd_packaged(mis);
> > > >        case MIG_CMD_POSTCOPY_ADVISE:
> > > > -        return loadvm_postcopy_handle_advise(mis);
> > > > +        return loadvm_postcopy_handle_advise(mis, len);
> > > >        case MIG_CMD_POSTCOPY_LISTEN:
> > > >            return loadvm_postcopy_handle_listen(mis);
> > > 
> 
> 
> -- 
> Best regards,
> Vladimir
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK