Re: [PATCH] x86: work around clang IAS bug referencing __force_order

[Arvind Sankar]

On Sat, Aug 22, 2020 at 02:08:27PM -0700, Linus Torvalds wrote:
> However, in this case, can we just leave that old "__force_order" hack
> alone, and to work around the clang thing, just make a dummy
> definition of it anyway.
> 
> Alternatively, just use the memory clobber. We use memory clobbers
> elsewhere in inline asms to make sure they are serialized, it's not
> normally a huge problem. Both clang and gcc should be smart enough to
> know that a memory clobber doesn't matter for things like local
> variables etc that might be on stack but have never had their address
> taken.
> 
> Or are there other cases than that particular __force_order thing that
> people now worry about?
> 
>              Linus

Actually, is a memory clobber required for correctness? Memory accesses
probably shouldn't be reordered across a CRn write. Is asm volatile
enough to stop that or do you need a memory clobber?

Replacing force_order with memory clobber introduces a few extra
instructions (testing with defconfig), but only in x86-64
hibernate/reboot/sleep code and early_ioremap_init on x86-32.