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

On Mon, Jun 21, 2021 at 07:46:03PM +1000, Michael Ellerman wrote:

Nathan Chancellor [off-list ref] writes:

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On 6/20/2021 4:59 PM, Nicholas Piggin wrote:

quoted

Excerpts from kernel test robot's message of April 3, 2021 8:47 pm:

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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 <redacted>

All errors (new ones prefixed by >>):

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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

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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.

The diff is pretty large so I have attached it here along with the full
disassembly of the files before and after the patch I am about to send.
I will reply to this message so the history is there.

Cheers,
Nathan