Re: [PATCH V2] memory: RAM_NAMED_FILE flag

[Peter Xu]

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