Re: [PATCH V2] migration: ram block cpr blockers

[Peter Xu <peterx@redhat.com>]

On Fri, Feb 14, 2025 at 04:44:40PM -0500, Steven Sistare wrote:
> On 2/14/2025 4:21 PM, David Hildenbrand wrote:
> > On 14.02.25 21:12, Steve Sistare wrote:
> > > Unlike cpr-reboot mode, cpr-transfer mode cannot save volatile ram blocks
> > > in the migration stream file and recreate them later, because the physical
> > > memory for the blocks is pinned and registered for vfio.  Add a blocker
> > > for volatile ram blocks.
> > > 
> > > Also add a blocker for RAM_GUEST_MEMFD.  Preserving guest_memfd may be
> > > sufficient for CPR, but it has not been tested yet.
> > > 
> > > Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> > > Reviewed-by: Fabiano Rosas <farosas@suse.de>
> > > ---
> > >   include/exec/memory.h   |  3 +++
> > >   include/exec/ramblock.h |  1 +
> > >   migration/savevm.c      |  2 ++
> > >   system/physmem.c        | 68 +++++++++++++++++++++++++++++++++++++++++++++++++
> > >   4 files changed, 74 insertions(+)
> > > 
> > > diff --git a/include/exec/memory.h b/include/exec/memory.h
> > > index 9f73b59..ea5d33a 100644
> > > --- a/include/exec/memory.h
> > > +++ b/include/exec/memory.h
> > > @@ -3184,6 +3184,9 @@ bool ram_block_discard_is_disabled(void);
> > >    */
> > >   bool ram_block_discard_is_required(void);
> > > +void ram_block_add_cpr_blocker(RAMBlock *rb, Error **errp);
> > > +void ram_block_del_cpr_blocker(RAMBlock *rb);
> > > +
> > >   #endif
> > >   #endif
> > > diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
> > > index 0babd10..64484cd 100644
> > > --- a/include/exec/ramblock.h
> > > +++ b/include/exec/ramblock.h
> > > @@ -39,6 +39,7 @@ struct RAMBlock {
> > >       /* RCU-enabled, writes protected by the ramlist lock */
> > >       QLIST_ENTRY(RAMBlock) next;
> > >       QLIST_HEAD(, RAMBlockNotifier) ramblock_notifiers;
> > > +    Error *cpr_blocker;
> > >       int fd;
> > >       uint64_t fd_offset;
> > >       int guest_memfd;
> > > diff --git a/migration/savevm.c b/migration/savevm.c
> > > index bc375db..85a3559 100644
> > > --- a/migration/savevm.c
> > > +++ b/migration/savevm.c
> > > @@ -3315,12 +3315,14 @@ void vmstate_register_ram(MemoryRegion *mr, DeviceState *dev)
> > >       qemu_ram_set_idstr(mr->ram_block,
> > >                          memory_region_name(mr), dev);
> > >       qemu_ram_set_migratable(mr->ram_block);
> > > +    ram_block_add_cpr_blocker(mr->ram_block, &error_fatal);
> > >   }
> > >   void vmstate_unregister_ram(MemoryRegion *mr, DeviceState *dev)
> > >   {
> > >       qemu_ram_unset_idstr(mr->ram_block);
> > >       qemu_ram_unset_migratable(mr->ram_block);
> > > +    ram_block_del_cpr_blocker(mr->ram_block);
> > >   }
> > >   void vmstate_register_ram_global(MemoryRegion *mr)
> > > diff --git a/system/physmem.c b/system/physmem.c
> > > index 67c9db9..c416068 100644
> > > --- a/system/physmem.c
> > > +++ b/system/physmem.c
> > > @@ -70,7 +70,10 @@
> > >   #include "qemu/pmem.h"
> > > +#include "qapi/qapi-types-migration.h"
> > > +#include "migration/blocker.h"
> > >   #include "migration/cpr.h"
> > > +#include "migration/options.h"
> > >   #include "migration/vmstate.h"
> > >   #include "qemu/range.h"
> > > @@ -1899,6 +1902,14 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
> > >               qemu_mutex_unlock_ramlist();
> > >               goto out_free;
> > >           }
> > > +
> > > +        error_setg(&new_block->cpr_blocker,
> > > +                   "Memory region %s uses guest_memfd, "
> > > +                   "which is not supported with CPR.",
> > > +                   memory_region_name(new_block->mr));
> > > +        migrate_add_blocker_modes(&new_block->cpr_blocker, errp,
> > > +                                  MIG_MODE_CPR_TRANSFER,
> > > +                                  -1);
> > >       }
> > >       ram_size = (new_block->offset + new_block->max_length) >> TARGET_PAGE_BITS;
> > > @@ -4059,3 +4070,60 @@ bool ram_block_discard_is_required(void)
> > >       return qatomic_read(&ram_block_discard_required_cnt) ||
> > >              qatomic_read(&ram_block_coordinated_discard_required_cnt);
> > >   }
> > > +
> > > +/*
> > > + * Return true if ram contents would be lost during CPR.  Do not exclude rom,
> > > + * because the rom file could change in new QEMU.
> > > + */
> > > +static bool ram_is_volatile(RAMBlock *rb)
> > 
> > Can we call this
> > 
> > ram_is_cpr_compatible() / ram_is_cpr_incompatible() or sth. instead?
> > 
> > Talking about RAM and "volatile" is misleading, and the function is specific to CPR already (e.g., comment :) ).
> 
> Will do, thanks - steve

With that, feel free to take:

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu