On Mon, Jun 21, 2021 at 11:46 AM Michael Ellerman <mpe@ellerman.id.au> wrote:
> Nathan Chancellor <nathan@kernel.org> writes:
> > On 6/20/2021 4:59 PM, Nicholas Piggin wrote:
> >> Excerpts from kernel test robot's message of April 3, 2021 8:47 pm:
> >>>
> >>> vim +/kvmhv_enter_nested_guest +264 arch/powerpc/kvm/book3s_hv_nested.c
> >>
> >> Not much changed here recently. It's not that big a concern because it's
> >> only called in the KVM ioctl path, not in any deep IO paths or anything,
> >> and doesn't recurse. Might be a bit of inlining or stack spilling put it
> >> over the edge.
> >
> > It appears to be the fact that LLVM's PowerPC backend does not emit
> > efficient byteswap assembly:
> >
> > https://github.com/ClangBuiltLinux/linux/issues/1292
> >
> > https://bugs.llvm.org/show_bug.cgi?id=49610
> >
> >> powerpc does make it an error though, would be good to avoid that so the
> >> robot doesn't keep tripping over.
> >
> > Marking byteswap_pt_regs as 'noinline_for_stack' drastically reduces the
> > stack usage. If that is an acceptable solution, I can send it along
> > tomorrow.
>
> Yeah that should be OK.

That's fine with me as well.

> Can you post the before/after disassembly when
> you post the patch?
>
> It should just be two extra function calls, which shouldn't be enough
> overhead to be measurable.

The thing I remember is that the 'before' code here is some seriously bad
output from llvm, and it would be helpful to have someone get the compiler
to emit the correct powerpc byteswap instructions and avoid the excessive
stack spilling.

The warning here is just a symptom of a missed optimization and the
same thing probably happens elsewhere on powerpc, even if it doesn't
exceed the stack warning limit.

        Arnd