Re: [Qemu-devel] [PATCH] hostmem-file: warn when memory-backend-file, share=on and in incoming migration

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

* Catherine Ho (catherine.hecx@gmail.com) wrote:
> Hi Dr. David
> 
> On Wed, 10 Apr 2019 at 22:59, Dr. David Alan Gilbert <dgilbert@redhat.com>
> wrote:
> 
> > * Catherine Ho (catherine.hecx@gmail.com) wrote:
> > > Hi Igor
> > >
> > >
> > > On Mon, 8 Apr 2019 at 18:35, Igor Mammedov <imammedo@redhat.com> wrote:
> > >
> > > > On Sun,  7 Apr 2019 22:19:05 -0400
> > > > Catherine Ho <catherine.hecx@gmail.com> wrote:
> > > >
> > > > > Currently it is not forbidden to use "-object
> > > > memory-backend-file,share=on"
> > > > > and together with "-incoming". But after incoming migration is
> > finished,
> > > > > the memory-backend-file will be definitely written if share=on. So
> > the
> > > > > memory-backend-file can only be used once, but failed in the 2nd time
> > > > > incoming.
> > > > >
> > > > > Thus it gives a warning and the users can run the qemu if they really
> > > > > want to do it.
> > > >
> > > > Shouldn't we add a migration blocker in such a case instead of warning
> > > > and letting qemu run wild?
> > > >
> > >
> > > IMO, it doesn't need to block this. With share=on and -incoming, the user
> > > can
> > > still save the device memory state into memory-backend file again if
> > > ignore-shared
> > > capability is on.
> > >
> > > If we block this, the user can't use the ignore-shared capability in
> > > incoming
> > > migration.
> >
> > -incomign with share=on is a perfectly normal thing to do - it just
> > depends who you are sharing the file with and the lifetime of that
> > shared file.
> >
> > For example; if you're just running a qemu with vhost-user then you
> > use share=on - however wyou typically select the backend file as
> > a new file from /dev/shm - it's not a file that you previously migrated
> > to.
> >
> Thanks,
> but using a new file from /dev/shm means kernel will start from
> start_kernel or early? Is it different from the x-ignore-shared case?
> If we remove the share=on in incoming migration, all the writting
> of ram will not be flush into the memory backend file. Thus we
> can use the base memory backend file for ever.
> e.g.
> 1) save the vm like a snapshot, current ram state is "kernel
> has been started, systemd has been started"
> 2) restore it with -incoming and *no* share=on flag
> 3) restore it with -incoming and *no* share=on again...
> In contrary, if we use share=on, the base backend file will
> be written at once after 1st time incoming.
> 
> So, IMO, no "share=on" is the proper usage of incoming migration
> when ignore-shared is on.
> Please correct me if sth is wrong, thanks:)

OK, I see what you're trying to do - you mean for the 'snapshotting'
case;  but that's not the only use.  Another use is for being able to
do a very quick upgrade of the running qemu to a new qemu binary
version; and in that case you want to be able to write to the shared
file so that you can repeatedly do the quick migrate.

Dave

> 
> B.R.
> Catherine
> 
> 
> > QEMU has no way to know about the provenance of the shared file it's
> > been given, so we can't really warn people about it.
> >
> > Dave
> >
> > > B.R.
> > > Catherine
> > >
> > > >
> > > >
> > > > > Signed-off-by: Catherine Ho <catherine.hecx@gmail.com>
> > > > > ---
> > > > >  backends/hostmem-file.c | 11 +++++++++++
> > > > >  1 file changed, 11 insertions(+)
> > > > >
> > > > > diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> > > > > index 37ac6445d2..59429ee0b4 100644
> > > > > --- a/backends/hostmem-file.c
> > > > > +++ b/backends/hostmem-file.c
> > > > > @@ -16,6 +16,7 @@
> > > > >  #include "sysemu/hostmem.h"
> > > > >  #include "sysemu/sysemu.h"
> > > > >  #include "qom/object_interfaces.h"
> > > > > +#include "migration/migration.h"
> > > > >
> > > > >  /* hostmem-file.c */
> > > > >  /**
> > > > > @@ -79,6 +80,16 @@ file_backend_memory_alloc(HostMemoryBackend
> > *backend,
> > > > Error **errp)
> > > > >          }
> > > > >      }
> > > > >
> > > > > +    /*
> > > > > +     * In ignore shared case, if share=on for host memory backend
> > file,
> > > > > +     * the ram might be written after incoming process is finished.
> > Thus
> > > > > +     * the memory backend can't be reused for 2nd/3rd... incoming
> > > > > +     */
> > > > > +    if (backend->share && migrate_ignore_shared()
> > > > > +                       && runstate_check(RUN_STATE_INMIGRATE))
> > > > > +        warn_report("share=on for memory backend file might be "
> > > > > +                        "conflicted with incoming in ignore shared
> > > > case");
> > > > > +
> > > > >      backend->force_prealloc = mem_prealloc;
> > > > >      name = host_memory_backend_get_name(backend);
> > > > >      memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
> > > >
> > > >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK