Re: [PATCH 1/2] riscv: Add memmove string operation.

[Nick Hu]

Hi Paul,

On Wed, Aug 14, 2019 at 10:03:39AM -0700, Paul Walmsley wrote:
> Hi Nick,
> 
> On Wed, 14 Aug 2019, Nick Hu wrote:
> 
> > On Wed, Aug 14, 2019 at 10:22:15AM +0800, Paul Walmsley wrote:
> > > On Tue, 13 Aug 2019, Palmer Dabbelt wrote:
> > > 
> > > > On Mon, 12 Aug 2019 08:04:46 PDT (-0700), Christoph Hellwig wrote:
> > > > > On Wed, Aug 07, 2019 at 03:19:14PM +0800, Nick Hu wrote:
> > > > > > There are some features which need this string operation for compilation,
> > > > > > like KASAN. So the purpose of this porting is for the features like KASAN
> > > > > > which cannot be compiled without it.
> > > > > > 
> > > > > > KASAN's string operations would replace the original string operations and
> > > > > > call for the architecture defined string operations. Since we don't have
> > > > > > this in current kernel, this patch provides the implementation.
> > > > > > 
> > > > > > This porting refers to the 'arch/nds32/lib/memmove.S'.
> > > > > 
> > > > > This looks sensible to me, although my stringop asm is rather rusty,
> > > > > so just an ack and not a real review-by:
> > > > 
> > > > FWIW, we just write this in C everywhere else and rely on the compiler to
> > > > unroll the loops.  I always prefer C to assembly when possible, so I'd prefer
> > > > if we just adopt the string code from newlib.  We have a RISC-V-specific
> > > > memcpy in there, but just use the generic memmove.
> > > > 
> > > > Maybe the best bet here would be to adopt the newlib memcpy/memmove as generic
> > > > Linux functions?  They're both in C so they should be fine, and they both look
> > > > faster than what's in lib/string.c.  Then everyone would benefit and we don't
> > > > need this tricky RISC-V assembly.  Also, from the look of it the newlib code
> > > > is faster because the inner loop is unrolled.
> > > 
> > > There's a generic memmove implementation in the kernel already:
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/string.h#n362
> > > 
> > > Nick, could you tell us more about why the generic memmove() isn't 
> > > suitable?
> > 
> > KASAN has its own string operations(memcpy/memmove/memset) because it needs to
> > hook some code to check memory region. It would undefined the original string
> > operations and called the string operations with the prefix '__'. But the
> > generic string operations didn't declare with the prefix. Other archs with
> > KASAN support like arm64 and xtensa all have their own string operations and
> > defined with the prefix.
> 
> Thanks for the explanation.  What do you think about Palmer's idea to 
> define a generic C set of KASAN string operations, derived from the newlib 
> code?
> 
> 
> - Paul

That sounds good to me. But it should be another topic. We need to investigate
it further about replacing something generic and fundamental in lib/string.c
with newlib C functions.  Some blind spots may exist.  So I suggest, let's
consider KASAN for now.

Nick