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physmem: Support coordinated discarding of RAM with guest_memfd

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A new field, attributes, was introduced in RAMBlock to link to a
RamBlockAttributes object, which centralizes all guest_memfd related
information (such as fd and status bitmap) within a RAMBlock.

Create and initialize the RamBlockAttributes object upon ram_block_add().
Meanwhile, register the object in the target RAMBlock's MemoryRegion.
After that, guest_memfd-backed RAMBlock is associated with the
RamDiscardManager interface, and the users can execute RamDiscardManager
specific handling. For example, VFIO will register the
RamDiscardListener and get notifications when the state_change() helper
invokes.

As coordinate discarding of RAM with guest_memfd is now supported, only
block uncoordinated discard.

Tested-by: Alexey Kardashevskiy <aik@amd.com>
Reviewed-by: Alexey Kardashevskiy <aik@amd.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Link: https://lore.kernel.org/r/20250612082747.51539-6-chenyi.qiang@intel.com
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Chenyi Qiang 2025-06-12 16:27:46 +08:00 • committed by Peter Xu
parent 5d6483edaa
commit 2fde3fb916
3 changed files with 31 additions and 2 deletions
Download patch file Download diff file Expand all files Collapse all files

9
accel/kvm/kvm-all.c
View file

@ -3091,6 +3091,15 @@ int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private)
addr = memory_region_get_ram_ptr(mr) + section.offset_within_region;
rb = qemu_ram_block_from_host(addr, false, &offset);
ret = ram_block_attributes_state_change(RAM_BLOCK_ATTRIBUTES(mr->rdm),
offset, size, to_private);
if (ret) {
error_report("Failed to notify the listener the state change of "
"(0x%"HWADDR_PRIx" + 0x%"HWADDR_PRIx") to %s",
start, size, to_private ? "private" : "shared");
goto out_unref;
}
if (to_private) {
if (rb->page_size != qemu_real_host_page_size()) {
/*

1
include/system/ramblock.h
View file

@ -46,6 +46,7 @@ struct RAMBlock {
int fd;
uint64_t fd_offset;
int guest_memfd;
RamBlockAttributes *attributes;
size_t page_size;
/* dirty bitmap used during migration */
unsigned long *bmap;

23
system/physmem.c
View file

@ -1916,7 +1916,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
}
assert(new_block->guest_memfd < 0);
ret = ram_block_discard_require(true);
ret = ram_block_coordinated_discard_require(true);
if (ret < 0) {
error_setg_errno(errp, -ret,
"cannot set up private guest memory: discard currently blocked");
@ -1931,6 +1931,24 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
goto out_free;
}
/*
* The attribute bitmap of the RamBlockAttributes is default to
* discarded, which mimics the behavior of kvm_set_phys_mem() when it
* calls kvm_set_memory_attributes_private(). This leads to a brief
* period of inconsistency between the creation of the RAMBlock and its
* mapping into the physical address space. However, this is not
* problematic, as no users rely on the attribute status to perform
* any actions during this interval.
*/
new_block->attributes = ram_block_attributes_create(new_block);
if (!new_block->attributes) {
error_setg(errp, "Failed to create ram block attribute");
close(new_block->guest_memfd);
ram_block_coordinated_discard_require(false);
qemu_mutex_unlock_ramlist();
goto out_free;
}
/*
* Add a specific guest_memfd blocker if a generic one would not be
* added by ram_block_add_cpr_blocker.
@ -2287,8 +2305,9 @@ static void reclaim_ramblock(RAMBlock *block)
}
if (block->guest_memfd >= 0) {
ram_block_attributes_destroy(block->attributes);
close(block->guest_memfd);
ram_block_discard_require(false);
ram_block_coordinated_discard_require(false);
}
g_free(block);