On Tue, Feb 07, 2023 at 11:03:33AM -0800, Steve Sistare wrote:
> migrate_ignore_shared() is an optimization that avoids copying memory
> that is visible and can be mapped on the target. However, a
> memory-backend-ram or a memory-backend-memfd block with the RAM_SHARED
> flag set is not migrated when migrate_ignore_shared() is true. This is
> wrong, because the block has no named backing store, and its contents will
> be lost. To fix, ignore shared memory iff it is a named file. Define a
> new flag RAM_NAMED_FILE to distinguish this case.
There's also TYPE_MEMORY_BACKEND_EPC. Reading the commit message it seems
it can still be used in similar ways. Pasting commit message from c6c0232:
Because of its unique requirements, Linux manages EPC separately from
normal memory. Similar to memfd, the device /dev/sgx_vepc can be
opened to obtain a file descriptor which can in turn be used to mmap()
EPC memory.
I'm not sure whether it means that should apply for RAM_NAMED_FILE too,
neither do I think it's super important.. Still better to define it
properly.
Another comment is, AFAIK this patch will modify senamtics of the old
capability "x-ignore-shared". But I'd say in a sensible way. Maybe worth
directly modify qapi/migration.json to reflect it (especially it's x-
prefixed) to something like:
# @x-ignore-shared: If enabled, QEMU will not migrate named shared memory
# (since 4.0)
Thanks,
--
Peter Xu
