Re: [PATCH v1 4/4] virtio-mem: Support "x-ignore-shared" migration

[David Hildenbrand]

On 20.06.23 15:06, Michael S. Tsirkin wrote:
> On Tue, Jun 20, 2023 at 03:03:54PM +0200, David Hildenbrand wrote:
> > To achieve desired "x-ignore-shared" functionality, we should not
> > discard all RAM when realizing the device and not mess with
> > preallocation/postcopy when loading device state. In essence, we should
> > not touch RAM content.
> >
> > As "x-ignore-shared" gets set after realizing the device, we cannot
> > rely on that. Let's simply skip discarding of RAM on incoming migration.
> > Note that virtio_mem_post_load() will call
> > virtio_mem_restore_unplugged() -- unless "x-ignore-shared" is set. So
> > once migration finished we'll have a consistent state.
> >
> > The initial system reset will also not discard any RAM, because
> > virtio_mem_unplug_all() will not call virtio_mem_unplug_all() when no
> > memory is plugged (which is the case before loading the device state).
> >
> > Note that something like VM templating -- see commit b17fbbe55cba
> > ("migration: allow private destination ram with x-ignore-shared") -- is
> > currently incompatible with virtio-mem and ram_block_discard_range() will
> > warn in case a private file mapping is supplied by virtio-mem.
> >
> > For VM templating with virtio-mem, it makes more sense to either
> > (a) Create the template without the virtio-mem device and hotplug a
> >      virtio-mem device to the new VM instances using proper own memory
> >      backend.
> > (b) Use a virtio-mem device that doesn't provide any memory in the
> >      template (requested-size=0) and use private anonymous memory.
> >
> > Signed-off-by: David Hildenbrand <david@redhat.com>
> > ---
> >   hw/virtio/virtio-mem.c | 47 ++++++++++++++++++++++++++++++++++--------
> >   1 file changed, 38 insertions(+), 9 deletions(-)
> >
> > diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> > index 9f6169af32..b013dfbaf0 100644
> > --- a/hw/virtio/virtio-mem.c
> > +++ b/hw/virtio/virtio-mem.c
> > @@ -18,6 +18,7 @@
> >   #include "sysemu/numa.h"
> >   #include "sysemu/sysemu.h"
> >   #include "sysemu/reset.h"
> > +#include "sysemu/runstate.h"
> >   #include "hw/virtio/virtio.h"
> >   #include "hw/virtio/virtio-bus.h"
> >   #include "hw/virtio/virtio-access.h"
> > @@ -886,11 +887,23 @@ static void virtio_mem_device_realize(DeviceState *dev, 
> > Error **errp)
> >           return;
> >       }
> >   
> > -    ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
> > -    if (ret) {
> > -        error_setg_errno(errp, -ret, "Unexpected error discarding RAM");
> > -        ram_block_coordinated_discard_require(false);
> > -        return;
> > +    /*
> > +     * We don't know at this point whether shared RAM is migrated using
> > +     * QEMU or migrated using the file content. "x-ignore-shared" will be
> > +     * configurated
>
> configurated == configured?

Thanks!

--
Cheers,

David / dhildenb