Re: [PATCH v4 04/13] memory: Introduce generic state change parent class for RamDiscardManager

[Chenyi Qiang <chenyi.qiang@intel.com>]

On 4/9/2025 5:56 PM, Alexey Kardashevskiy wrote:
> 
> 
> On 7/4/25 17:49, Chenyi Qiang wrote:
>> RamDiscardManager is an interface used by virtio-mem to adjust VFIO
>> mappings in relation to VM page assignment. It manages the state of
>> populated and discard for the RAM. To accommodate future scnarios for
>> managing RAM states, such as private and shared states in confidential
>> VMs, the existing RamDiscardManager interface needs to be generalized.
>>
>> Introduce a parent class, GenericStateManager, to manage a pair of
> 
> "GenericState" is the same as "State" really. Call it RamStateManager.

OK to me.

> 
> 
> 
>> opposite states with RamDiscardManager as its child. The changes include
>> - Define a new abstract class GenericStateChange.
>> - Extract six callbacks into GenericStateChangeClass and allow the child
>>    classes to inherit them.
>> - Modify RamDiscardManager-related helpers to use GenericStateManager
>>    ones.
>> - Define a generic StatChangeListener to extract fields from
> 
> "e" missing in StateChangeListener.

Fixed. Thanks.

> 
>>    RamDiscardManager listener which allows future listeners to embed it
>>    and avoid duplication.
>> - Change the users of RamDiscardManager (virtio-mem, migration, etc.) to
>>    switch to use GenericStateChange helpers.
>>
>> It can provide a more flexible and resuable framework for RAM state
>> management, facilitating future enhancements and use cases.
> 
> I fail to see how new interface helps with this. RamDiscardManager
> manipulates populated/discarded. It would make sense may be if the new
> class had more bits per page, say private/shared/discarded but it does
> not. And PrivateSharedManager cannot coexist with RamDiscard. imho this
> is going in a wrong direction.

I think we have two questions here:

1. whether we should define an abstract parent class and distinguish the
RamDiscardManager and PrivateSharedManager?

I vote for this. First, After making the distinction, the
PrivateSharedManager won't go into the RamDiscardManager path which
PrivateSharedManager may have not supported yet. e.g. the migration
related path. In addtional, we can extend the PrivateSharedManager for
specific handling, e.g. the priority listener, state_change() callback.

2. How we should abstract the parent class?

I think this is the problem. My current implementation extracts all the
callbacks in RamDiscardManager into the parent class and call them
state_set and state_clear, which can only manage a pair of opposite
states. As you mentioned, there could be private/shared/discarded three
states in the future, which is not compatible with current design. Maybe
we can make the parent class more generic, e.g. only extract the
register/unregister_listener() into it.

> 
> 
>>
>> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
>> ---
>> Changes in v4:
>>      - Newly added.
>> ---
>>   hw/vfio/common.c        |  30 ++--
>>   hw/virtio/virtio-mem.c  |  95 ++++++------
>>   include/exec/memory.h   | 313 ++++++++++++++++++++++------------------
>>   migration/ram.c         |  16 +-
>>   system/memory.c         | 106 ++++++++------
>>   system/memory_mapping.c |   6 +-
>>   6 files changed, 310 insertions(+), 256 deletions(-)
>>
>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
>> index f7499a9b74..3172d877cc 100644
>> --- a/hw/vfio/common.c
>> +++ b/hw/vfio/common.c
>> @@ -335,9 +335,10 @@ out:
>>       rcu_read_unlock();
>>   }
>>   -static void vfio_ram_discard_notify_discard(RamDiscardListener *rdl,
>> +static void vfio_ram_discard_notify_discard(StateChangeListener *scl,
>>                                               MemoryRegionSection
>> *section)
>>   {
>> +    RamDiscardListener *rdl = container_of(scl, RamDiscardListener,
>> scl);
>>       VFIORamDiscardListener *vrdl = container_of(rdl,
>> VFIORamDiscardListener,
>>                                                   listener);
>>       VFIOContainerBase *bcontainer = vrdl->bcontainer;
>> @@ -353,9 +354,10 @@ static void
>> vfio_ram_discard_notify_discard(RamDiscardListener *rdl,
>>       }
>>   }
>>   -static int vfio_ram_discard_notify_populate(RamDiscardListener *rdl,
>> +static int vfio_ram_discard_notify_populate(StateChangeListener *scl,
>>                                               MemoryRegionSection
>> *section)
>>   {
>> +    RamDiscardListener *rdl = container_of(scl, RamDiscardListener,
>> scl);
>>       VFIORamDiscardListener *vrdl = container_of(rdl,
>> VFIORamDiscardListener,
>>                                                   listener);
> 
> VFIORamDiscardListener *vrdl = container_of(scl, VFIORamDiscardListener,
> listener.scl) and drop @ rdl? Thanks,

Modified. Thanks!

> 
>