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

[Steven Sistare <steven.sistare@oracle.com>]

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