Re: arch/powerpc/kvm/book3s_hv_nested.c:264:6: error: stack frame size of 2304 bytes in function 'kvmhv_enter_nested_guest'

[Michael Ellerman <mpe@ellerman.id.au>]

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:
>>> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
>>> head:   d93a0d43e3d0ba9e19387be4dae4a8d5b175a8d7
>>> commit: 97e4910232fa1f81e806aa60c25a0450276d99a2 linux/compiler-clang.h: define HAVE_BUILTIN_BSWAP*
>>> date:   3 weeks ago
>>> config: powerpc64-randconfig-r006-20210403 (attached as .config)
>>> compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 0fe8af94688aa03c01913c2001d6a1a911f42ce6)
>>> reproduce (this is a W=1 build):
>>>          wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>>          chmod +x ~/bin/make.cross
>>>          # install powerpc64 cross compiling tool for clang build
>>>          # apt-get install binutils-powerpc64-linux-gnu
>>>          # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=97e4910232fa1f81e806aa60c25a0450276d99a2
>>>          git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>>>          git fetch --no-tags linus master
>>>          git checkout 97e4910232fa1f81e806aa60c25a0450276d99a2
>>>          # save the attached .config to linux build tree
>>>          COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=powerpc64
>>>
>>> If you fix the issue, kindly add following tag as appropriate
>>> Reported-by: kernel test robot <lkp@intel.com>
>>>
>>> All errors (new ones prefixed by >>):
>>>
>>>>> arch/powerpc/kvm/book3s_hv_nested.c:264:6: error: stack frame size of 2304 bytes in function 'kvmhv_enter_nested_guest' [-Werror,-Wframe-larger-than=]
>>>     long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu)
>>>          ^
>>>     1 error generated.
>>>
>>>
>>> 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. 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.

cheers