Re: [PATCH V2] memory: RAM_NAMED_FILE flag

[Steven Sistare]

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