On 2/8/2023 2:22 PM, Peter Xu wrote: > 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.
Hi Peter, I will rebase to the tip, modify the wording, and repost. Can I add your RB? - Steve
