Re: [RFC] Add asm constraint modifier to mark strict memory accesses

[Jeff Law <law at redhat dot com>]

On 09/18/14 05:19, Yury Gribov wrote:
> > Would that modifier mean that the inline asm is unconditionally reading
> > resp. writing that memory? "m"/"=m" right now is always about might
> > read or might write, not must.
>
> Yes, that's what I had in mind. Many inline asms (at least in kernel) do
> read memory region unconditionally.
That's precisely what I'd expect such a modifier to mean.  Right now 
memory modifiers are strictly "may" but I can see a use case for "must".

I think the question is will the kernel or glibc folks use that new 
capability and if so, do we get a significant improvement in the amount 
of checking we can do.    So I think both those groups need to be looped 
into this conversation.

From an implementation standpoint, are you thinking a different 
modifier (my first choice)?  That wouldn't allow us to say something 
like the first X bytes of this memory region are written and the 
remaining Y bytes may be written, but I suspect that's not a use case 
we're likely to care about.

jeff