On 5/26/2025 5:01 PM, David Hildenbrand wrote:
> On 20.05.25 12:28, Chenyi Qiang wrote:
>> Commit 852f0048f3 ("RAMBlock: make guest_memfd require uncoordinated
>> discard") highlighted that subsystems like VFIO may disable RAM block
>> discard. However, guest_memfd relies on discard operations for page
>> conversion between private and shared memory, potentially leading to
>> stale IOMMU mapping issue when assigning hardware devices to
>> confidential VMs via shared memory. To address this and allow shared
>> device assignement, it is crucial to ensure VFIO system refresh its
>> IOMMU mappings.
>>
>> RamDiscardManager is an existing interface (used by virtio-mem) to
>> adjust VFIO mappings in relation to VM page assignment. Effectively page
>> conversion is similar to hot-removing a page in one mode and adding it
>> back in the other. Therefore, similar actions are required for page
>> conversion events. Introduce the RamDiscardManager to guest_memfd to
>> facilitate this process.
>>
>> Since guest_memfd is not an object, it cannot directly implement the
>> RamDiscardManager interface. Implementing it in HostMemoryBackend is
>> not appropriate because guest_memfd is per RAMBlock, and some RAMBlocks
>> have a memory backend while others do not. Notably, virtual BIOS
>> RAMBlocks using memory_region_init_ram_guest_memfd() do not have a
>> backend.
>>
>> To manage RAMBlocks with guest_memfd, define a new object named
>> RamBlockAttribute to implement the RamDiscardManager interface. This
>> object can store the guest_memfd information such as bitmap for shared
>> memory, and handles page conversion notification. In the context of
>> RamDiscardManager, shared state is analogous to populated and private
>> state is treated as discard. The memory state is tracked at the host
>> page size granularity, as minimum memory conversion size can be one page
>> per request. Additionally, VFIO expects the DMA mapping for a specific
>> iova to be mapped and unmapped with the same granularity. Confidential
>> VMs may perform partial conversions, such as conversions on small
>> regions within larger regions. To prevent such invalid cases and until
>> cut_mapping operation support is available, all operations are performed
>> with 4K granularity.
>>
>> Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
>> ---
>> Changes in v5:
>> - Revert to use RamDiscardManager interface instead of introducing
>> new hierarchy of class to manage private/shared state, and keep
>> using the new name of RamBlockAttribute compared with the
>> MemoryAttributeManager in v3.
>> - Use *simple* version of object_define and object_declare since the
>> state_change() function is changed as an exported function instead
>> of a virtual function in later patch.
>> - Move the introduction of RamBlockAttribute field to this patch and
>> rename it to ram_shared. (Alexey)
>> - call the exit() when register/unregister failed. (Zhao)
>> - Add the ram-block-attribute.c to Memory API related part in
>> MAINTAINERS.
>>
>> Changes in v4:
>> - Change the name from memory-attribute-manager to
>> ram-block-attribute.
>> - Implement the newly-introduced PrivateSharedManager instead of
>> RamDiscardManager and change related commit message.
>> - Define the new object in ramblock.h instead of adding a new file.
>>
>> Changes in v3:
>> - Some rename (bitmap_size->shared_bitmap_size,
>> first_one/zero_bit->first_bit, etc.)
>> - Change shared_bitmap_size from uint32_t to unsigned
>> - Return mgr->mr->ram_block->page_size in get_block_size()
>> - Move set_ram_discard_manager() up to avoid a g_free() in failure
>> case.
>> - Add const for the memory_attribute_manager_get_block_size()
>> - Unify the ReplayRamPopulate and ReplayRamDiscard and related
>> callback.
>>
>> Changes in v2:
>> - Rename the object name to MemoryAttributeManager
>> - Rename the bitmap to shared_bitmap to make it more clear.
>> - Remove block_size field and get it from a helper. In future, we
>> can get the page_size from RAMBlock if necessary.
>> - Remove the unncessary "struct" before GuestMemfdReplayData
>> - Remove the unncessary g_free() for the bitmap
>> - Add some error report when the callback failure for
>> populated/discarded section.
>> - Move the realize()/unrealize() definition to this patch.
>> ---
>> MAINTAINERS | 1 +
>> include/system/ramblock.h | 20 +++
>> system/meson.build | 1 +
>> system/ram-block-attribute.c | 311 +++++++++++++++++++++++++++++++++++
>> 4 files changed, 333 insertions(+)
>> create mode 100644 system/ram-block-attribute.c
>>
>> diff --git a/MAINTAINERS b/MAINTAINERS
>> index 6dacd6d004..3b4947dc74 100644
>> --- a/MAINTAINERS
>> +++ b/MAINTAINERS
>> @@ -3149,6 +3149,7 @@ F: system/memory.c
>> F: system/memory_mapping.c
>> F: system/physmem.c
>> F: system/memory-internal.h
>> +F: system/ram-block-attribute.c
>> F: scripts/coccinelle/memory-region-housekeeping.cocci
>> Memory devices
>> diff --git a/include/system/ramblock.h b/include/system/ramblock.h
>> index d8a116ba99..09255e8495 100644
>> --- a/include/system/ramblock.h
>> +++ b/include/system/ramblock.h
>> @@ -22,6 +22,10 @@
>> #include "exec/cpu-common.h"
>> #include "qemu/rcu.h"
>> #include "exec/ramlist.h"
>> +#include "system/hostmem.h"
>> +
>> +#define TYPE_RAM_BLOCK_ATTRIBUTE "ram-block-attribute"
>> +OBJECT_DECLARE_SIMPLE_TYPE(RamBlockAttribute, RAM_BLOCK_ATTRIBUTE)
>> struct RAMBlock {
>> struct rcu_head rcu;
>> @@ -42,6 +46,8 @@ struct RAMBlock {
>> int fd;
>> uint64_t fd_offset;
>> int guest_memfd;
>> + /* 1-setting of the bitmap in ram_shared represents ram is shared */
>
> That comment looks misplaced, and the variable misnamed.
>
> The commet should go into RamBlockAttribute and the variable should
> likely be named "attributes".
>
> Also, "ram_shared" is not used at all in this patch, it should be moved
> into the corresponding patch.
I thought we only manage the private and shared attribute, so name it as
ram_shared. And in the future if managing other attributes, then rename
it to attributes. It seems I overcomplicated things.
>
>> + RamBlockAttribute *ram_shared;
>> size_t page_size;
>> /* dirty bitmap used during migration */
>> unsigned long *bmap;
>> @@ -91,4 +97,18 @@ struct RAMBlock {
>> ram_addr_t postcopy_length;
>> };
>> +struct RamBlockAttribute {
>
> Should this actually be "RamBlockAttributes" ?
Yes. To match with variable name "attributes", it can be renamed as
RamBlockAttributes.
>
>> + Object parent;
>> +
>> + MemoryRegion *mr;
>
>
> Should we link to the parent RAMBlock instead, and lookup the MR from
> there?
Good suggestion! It can also help to reduce the long arrow operation in
ram_block_attribute_get_block_size().
>
>
