Re: [PATCH] x86/memcpy: Introduce memcpy_mcsafe_fast

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sat, Apr 18, 2020 at 1:30 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
> Maybe I’m missing something obvious, but what’s the alternative?  The _mcsafe variants don’t just avoid the REP mess — they also tell the kernel that this particular access is recoverable via extable.

.. which they could easily do exactly the same way the user space
accessors do, just with a much simplified model that doesn't even care
about multiple sizes, since unaligned accesses weren't valid anyway.

The thing is, all of the MCS code has been nasty. There's no reason
for it what-so-ever that I can tell. The hardware has been so
incredibly broken that it's basically unusable, and most of the
software around it seems to have been about testing.

So I absolutely abhor that thing. Everything about that code has
screamed "yeah, we completely mis-designed the hardware, we're pushing
the problems into software, and nobody even uses it or can test it so
there's like 5 people who care".

And I'm pushing back on it, because I think that the least the code
can do is to at least be simple.

For example, none of those optimizations should exist. That function
shouldn't have been inline to begin with. And if it really really
matters from a performance angle that it was inline (which I doubt),
it shouldn't have looked like a memory copy, it should have looked
like "get_user()" (except without all the complications of actually
having to test addresses or worry about different sizes).

And it almost certainly shouldn't have been done in low-level asm
either. It could have been a single "read aligned word" interface
using an inline asm, and then everything else could have been done as
C code around it.

But no. The software side is almost as messy as the hardware side is.
I hate it. And since nobody sane can test it, and the broken hardware
is _so_ broken than nobody should ever use it, I have continually
pushed back against this kind of ugly nasty special code.

We know the writes can't fault, since they are buffered. So they
aren't special at all.

We know the acceptable reads for the broken hardware basically boil
down to a single simple word-size aligned read, so you need _one_
special inline asm for that. The rest of the cases can be handled by
masking and shifting if you really really need to - and done better
that way than with byte accesses anyway.

Then you have _one_ C file that implements everything using that
single operation (ok, if people absolutely want to do sizes, I guess
they can if they can just hide it in that one file), and you have one
header file that exposes the interfaces to it, and you're done.

And you strive hard as hell to not impact anything else, because you
know that the hardware is unacceptable until all those special rules
go away. Which they apparently finally have.

                Linus