On Wed, Feb 08, 2023 at 01:34:18PM -0500, Steven Sistare wrote: > 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.
My point was it looked like it should apply RAM_NAMED_FILE too, because it's also a named file. But.. I don't think another QEMU can share the same data if opening the same file. Based on my zero knowledge on it... I quickly looked up sgx_vepc_open() in the Linux impl where sgx_vepc.page_array plays a role of page cache iiuc, while it's private per vma even if VM_SHARED. So please ignore my comment.. > > > 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) I would use s/target/destination host/ because target can be misread as the VM itself (at least a general term in migration code base). Other than that looks good to me. Thanks, -- Peter Xu
