Re: [PATCH v2] powerpc: warn on emulation of dcbz instruction in kernel mode

[Segher Boessenkool <segher@kernel.crashing.org>]

On Sat, Aug 24, 2024 at 09:01:33AM +0000, LEROY Christophe wrote:
> Le 23/08/2024 à 21:19, Segher Boessenkool a écrit :
> > The memset() code itself could chech for the storage attributes, but
> > that is probably more expensive than just assuming the happy case.
> > Maybe someone could try it out though!
> 
> But is it only memset() the problem ?
> 
> dcbz instruction is also used in:
> - memcpy()
> - csum_partial_copy_generic()
> - clear_page()
> - copy_page()
> - clear_user()
> - copy_to_user()
> - copy_from_user()

That is just a handful of functions.  Not sure about the _user things,
and the _page things for that matter, but the rest is certainly
measurable in real-life conditions.  So if we can avoid the problems
completely, and cheaply, we probably should.

I'm not so sure about the cheaply though :-/

> Are these functions also used on DMA coherent memory ?

Most won't show up high on most profiles, heh.  Which you already
can see from the problem not being attacked yet: if it was so obviously
a problem, some people would have wanted to do something about it :-)


Segher