On 2/7/2023 3:23 PM, Peter Xu wrote: > 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.
The RAM_NAMED_FILE flag will be false for TYPE_MEMORY_BACKEND_EPC, so ramblock_is_ignored will return false, and the contents will be copied over the socket to the target, and the segment will be recreated there. However, perhaps I do not understand your point. > 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) Good idea. I propose: # @x-ignore-shared: If enabled, QEMU will not migrate shared memory that is # accessible on the target. (since 4.0) - Steve
