Re: [PATCH V2] memory: RAM_NAMED_FILE flag

[Peter Xu <peterx@redhat.com>]

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