On Mon, Jun 21, 2021 at 07:46:03PM +1000, Michael Ellerman wrote: > Nathan Chancellor <nathan@xxxxxxxxxx> 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@xxxxxxxxx> > >>> > >>> 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. 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
