On 20.10.25 12:32, Chenyi Qiang wrote:
On 10/17/2025 11:13 PM, David Hildenbrand wrote:
On 17.10.25 10:14, Chenyi Qiang wrote:
Currently, private memory and shared memory have different backend in
CoCo VMs. It is possible for users to specify the shared memory with
hugetlbfs backend while private memory with guest_memfd backend only
supports 4K page size. In this case, ram_block->page_size is different
from the host page size which will trigger the assertion when getting
block size. Relax the restriction to allow shared memory to use
hugetlbfs backend.
Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage
RAMBlock with guest_memfd")
Signed-off-by: Chenyi Qiang <[email protected]>
---
system/ram-block-attributes.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c
index 68e8a027032..0f39ccf9090 100644
--- a/system/ram-block-attributes.c
+++ b/system/ram-block-attributes.c
@@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const
RamBlockAttributes *attr)
* Because page conversion could be manipulated in the size of at least
4K
* or 4K aligned, Use the host page size as the granularity to track the
* memory attribute.
+ * When hugetlbfs is used as backend of shared memory, ram_block->page_size
+ * is different from host page size. So it is not appropriate to use
+ * ram_block->page_size here.
But are we sure everything else is working as expected and that this is not a
check that prevents other code from doing the wrong thing?
I think so. The block size must be 4K due to the page conversion could be in the size of
4K and we use "bitmap" to track the status.
Indeed.
I originally missed the case of hugetlb so added an assert() here. But it is
allowed to use hugetlb as shared memory backend
before shared device assignment patches were introduced.
I recall that punching holes was problematic as the VM shares/unshared 4k
chunks.
I can see the kvm_convert_memory() will skip ram_block_discard_range() if using
hugetlb backend.
It will cause the double-memory consumption (*). Any other problem?
Right.
What we should be doing is unifying the retrieval of the block size in
ram_block_attributes_create() as well. That's where we allocate it.
So either
a) Use qemu_real_host_page_size() everywhere.
b) Use ram_block_attributes_get_block_size() everywhere.
Could be done in a separate patch.
--
Cheers
David / dhildenb
