linux-kernel - Re: [PATCH v4 4/5] riscv: Vector checksum library

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Message-ID: <ZQN0SBwhYwDQHGF+@ghost>
Date:   Thu, 14 Sep 2023 16:59:52 -0400
From:   Charlie Jenkins <charlie@...osinc.com>
To:     Conor Dooley <conor@...nel.org>
Cc:     Conor Dooley <conor.dooley@...rochip.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Samuel Holland <samuel.holland@...ive.com>,
        David Laight <David.Laight@...lab.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH v4 4/5] riscv: Vector checksum library

On Thu, Sep 14, 2023 at 06:36:42PM +0100, Conor Dooley wrote:
> On Thu, Sep 14, 2023 at 01:29:18PM -0400, Charlie Jenkins wrote:
> > On Thu, Sep 14, 2023 at 05:29:29PM +0100, Conor Dooley wrote:
> > > On Thu, Sep 14, 2023 at 12:14:16PM -0400, Charlie Jenkins wrote:
> > > > On Thu, Sep 14, 2023 at 01:46:29PM +0100, Conor Dooley wrote:
> > > > > On Mon, Sep 11, 2023 at 03:57:14PM -0700, Charlie Jenkins wrote:
> > > > > > This patch is not ready for merge as vector support in the kernel is
> > > > > > limited. However, the code has been tested in QEMU so the algorithms
> > > > > > do work. This code requires the kernel to be compiled with C vector
> > > > > > support, but that is not yet possible. It is written in assembly
> > > > > > rather than using the GCC vector instrinsics because they did not
> > > > > > provide optimal code.
> > > > > > 
> > > > > > Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> > > > > > ---
> > > > > >  arch/riscv/lib/csum.c | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++
> > > > > >  1 file changed, 92 insertions(+)
> > > > > > 
> > > > > > diff --git a/arch/riscv/lib/csum.c b/arch/riscv/lib/csum.c
> > > > > > index 47d98c51bab2..eb4596fc7f5b 100644
> > > > > > --- a/arch/riscv/lib/csum.c
> > > > > > +++ b/arch/riscv/lib/csum.c
> > > > > > @@ -12,6 +12,10 @@
> > > > > >  
> > > > > >  #include <net/checksum.h>
> > > > > >  
> > > > > > +#ifdef CONFIG_RISCV_ISA_V
> > > > > > +#include <riscv_vector.h>
> > > > > 
> > > > > What actually includes this header, I don't see it in either Andy's
> > > > > in-kernel vector series or Bjorn's blake2 one.
> > > > > Can you link to the pre-requisites in your cover letter please.
> > > > > 
> > > > > Thanks,
> > > > > Conor.
> > > > 
> > > > It is defined here:
> > > > https://github.com/riscv-non-isa/rvv-intrinsic-doc/blob/main/doc/rvv-intrinsic-spec.adoc.
> > > > The header is for the vector intrinsics that are supported by llvm and
> > > > gcc.
> > > 
> > > Well, whatever you're doing with it does not work, producing 3600 or so
> > > fatal errors during compilation, all saying:
> > > ../arch/riscv/include/asm/checksum.h:14:10: fatal error: riscv_vector.h: No such file or directory
> > > 
> > > Do you have some makefile hack somewhere that's not part of this
> > > patchset? Also, I'm dumb, but can you show me where are the actual
> > > intrinsics are being used in this patch anyway? I just seem some
> > > types & asm.
> > > 
> > > Thanks,
> > > Conor.
> > > 
> > 
> > Intrinsics are needed for the vector types. Vector types are needed to
> > get the inline asm to select vector registers at compile time. I could
> > manually select vector registers to use but that is not ideal. In order
> > to get this to work, vector has to be enabled in the compiler. This
> > patch will not compile right now, but since people are working on vector
> > I was hoping that it would be possible in the future. Palmer recommended
> > that I just put up this patch for now since I had the code, but only the
> > non-vector versions should be candidates for release for now.
> 
> I see. I was pretty unclear to me anyway what the craic was, you should
> probably note that the build failures from here onwards are
> known-broken. If you want that header, I guess you probably need to
> have v set in -march?
> If so, the in-kernel vector patches that have been posted do not do that.
> Oh-so-far from an expert on what is a safe way to do these kinda things
> though, sadly.

It seems like more than just enabling v in the march will need to be
done. Because linux uses -nostdinc the header file won't be included.
After doing some research it also seems like llvm and gcc do not share
inline asm constraints. llvm is missing "vd" to specify a register that
is not a mask register. I think I will drop these vector patches for
now since there seems to be more work than I expected to get this
functional.

- Charlie