Re: AMDGPU and 16B stack alignment

[Arvind Sankar <nivedita@alum.mit.edu>]

On Tue, Oct 15, 2019 at 06:51:26PM -0700, Nick Desaulniers wrote:
> On Tue, Oct 15, 2019 at 1:26 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Tue, Oct 15, 2019 at 11:05:56AM -0700, Nick Desaulniers wrote:
> > > Hmmm...I would have liked to remove it outright, as it is an ABI
> > > mismatch that is likely to result in instability and non-fun-to-debug
> > > runtime issues in the future.  I suspect my patch does work for GCC
> > > 7.1+.  The question is: Do we want to either:
> > > 1. mark AMDGPU broken for GCC < 7.1, or
> > > 2. continue supporting it via stack alignment mismatch?
> > >
> > > 2 is brittle, and may break at any point in the future, but if it's
> > > working for someone it does make me feel bad to outright disable it.
> > > What I'd image 2 looks like is (psuedo code in a Makefile):
> > >
> > > if CC_IS_GCC && GCC_VERSION < 7.1:
> > >   set stack alignment to 16B and hope for the best
> > >
> > > So my diff would be amended to keep the stack alignment flags, but
> > > only to support GCC < 7.1.  And that assumes my change compiles with
> > > GCC 7.1+. (Looks like it does for me locally with GCC 8.3, but I would
> > > feel even more confident if someone with hardware to test on and GCC
> > > 7.1+ could boot test).
> > > --
> > > Thanks,
> > > ~Nick Desaulniers
> >
> > If we do keep it, would adding -mstackrealign make it more robust?
> > That's simple and will only add the alignment to functions that require
> > 16-byte alignment (at least on gcc).
> 
> I think there's also `-mincoming-stack-boundary=`.
> https://github.com/ClangBuiltLinux/linux/issues/735#issuecomment-540038017

Yes, but -mstackrealign looks like it's supported by clang as well.
> 
> >
> > Alternative is to use
> > __attribute__((force_align_arg_pointer)) on functions that might be
> > called from 8-byte-aligned code.
> 
> Which is hard to automate and easy to forget.  Likely a large diff to fix today.

Right, this is a no-go, esp to just fix old compilers.
> 
> >
> > It looks like -mstackrealign should work from gcc 5.3 onwards.
> 
> The kernel would generally like to support GCC 4.9+.
> 
> There's plenty of different ways to keep layering on duct tape and
> bailing wire to support differing ABIs, but that's just adding
> technical debt that will have to be repaid one day.  That's why the
> cleanest solution IMO is mark the driver broken for old toolchains,
> and use a code-base-consistent stack alignment.  Bending over
> backwards to support old toolchains means accepting stack alignment
> mismatches, which is in the "unspecified behavior" ring of the
> "undefined behavior" Venn diagram.  I have the same opinion on relying
> on explicitly undefined behavior.
> 
> I'll send patches for fixing up Clang, but please consider my strong
> advice to generally avoid stack alignment mismatches, regardless of
> compiler.
> --
> Thanks,
> ~Nick Desaulniers

What I suggested was in reference to your proposal for dropping the
-mpreferred-stack-boundary=4 for modern compilers, but keeping it for
<7.1. -mstackrealign would at least let 5.3 onwards be less likely to
break (and it doesn't error before then, I think it just doesn't
actually do anything, so no worse than now at least).

Simply dropping support for <7.1 would be cleanest, yes, but it sounds
like people don't want to go that far.