On 04.12.23 08:53, Xiaoyao Li wrote:
On 12/4/2023 3:35 PM, Xiaoyao Li wrote:
On 11/20/2023 5:56 PM, David Hildenbrand wrote:
On 16.11.23 03:56, Xiaoyao Li wrote:
On 11/16/2023 2:20 AM, David Hildenbrand wrote:
On 15.11.23 08:14, Xiaoyao Li wrote:
Commit d3a5038c461 ("exec: ram_block_discard_range") introduced
ram_block_discard_range() which grabs some code from
ram_discard_range(). However, during code movement, it changed
check of host_startaddr from qemu_host_page_size to rb->page_size.

When ramblock is back'ed by hugepage, it requires the startaddr to be
huge page size aligned, which is a overkill. e.g., TDX's
page conversion is done at 4KB granularity. Shared page is discarded
when it gets converts to private and when shared page back'ed by
hugepage it is going to fail on this check.

So change to alignment check back to qemu_host_page_size.

Signed-off-by: Xiaoyao Li <xiaoyao...@intel.com>
Changes in v3:
    - Newly added in v3;
    system/physmem.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/system/physmem.c b/system/physmem.c
index c56b17e44df6..8a4e42c7cf60 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -3532,7 +3532,7 @@ int ram_block_discard_range(RAMBlock *rb,
uint64_t start, size_t length)
        uint8_t *host_startaddr = rb->host + start;
-    if (!QEMU_PTR_IS_ALIGNED(host_startaddr, rb->page_size)) {
+    if (!QEMU_PTR_IS_ALIGNED(host_startaddr, qemu_host_page_size)) {

For your use cases, rb->page_size should always match

IIRC, we only set rb->page_size to different values for hugetlb. And
guest_memfd does not support hugetlb.

Even if QEMU is using THP, rb->page_size should 4k.

Please elaborate how you can actually trigger that. From what I recall,
guest_memfd is not compatible with hugetlb.

It's the shared memory that can be back'ed by hugetlb.

Serious question: does that configuration make any sense to support at
this point? I claim: no.

Later patch 9 introduces ram_block_convert_page(), which will discard
shared memory when it gets converted to private. TD guest can request
convert a 4K to private while the page is previously back'ed by hugetlb
as 2M shared page.

So you can call ram_block_discard_guest_memfd_range() on subpage
basis, but not ram_block_discard_range().

ram_block_convert_range() would have to thought that that
(questionable) combination of hugetlb for shmem and ordinary pages for
guest_memfd cannot discard shared memory.

And it probably shouldn't either way. There are other problems when
not using hugetlb along with preallocation.

If I understand correctly, preallocation needs to be enabled for
hugetlb. And in preallocation case, it doesn't need to discard memory.
Is it correct?

Yes The downside is that we'll end up with double-memory consumption. But if/how to optimize that in this case ca be left for future work.

The check in ram_block_discard_range() is correct, whoever ends up
calling it has to stop calling it.

  > So, I need add logic to ram_block_discard_page() that if the size of

Sorry, I made a typo.

Correct myself, s/ram_block_discard_page()/ram_block_convert_range()

Yes, just leave any shared memory backend that uses hugetlb alone.


David / dhildenb

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