Excerpts from Nathan Chancellor's message of June 22, 2021 4:24 am: > LLVM does not emit optimal byteswap assembly, which results in high > stack usage in kvmhv_enter_nested_guest() due to the inlining of > byteswap_pt_regs(). With LLVM 12.0.0: > > arch/powerpc/kvm/book3s_hv_nested.c:289:6: error: stack frame size of > 2512 bytes in function 'kvmhv_enter_nested_guest' [-Werror,-Wframe-larger-than=] > long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu) > ^ > 1 error generated. > > While this gets fixed in LLVM, mark byteswap_pt_regs() as > noinline_for_stack so that it does not get inlined and break the build > due to -Werror by default in arch/powerpc/. Not inlining saves > approximately 800 bytes with LLVM 12.0.0: > > arch/powerpc/kvm/book3s_hv_nested.c:290:6: warning: stack frame size of > 1728 bytes in function 'kvmhv_enter_nested_guest' [-Wframe-larger-than=] > long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu) > ^ > 1 warning generated. > > Link: https://github.com/ClangBuiltLinux/linux/issues/1292 > Link: https://bugs.llvm.org/show_bug.cgi?id=49610 > Link: https://lore.kernel.org/r/202104031853.vDT0Qjqj-lkp@xxxxxxxxx/ > Link: https://gist.github.com/ba710e3703bf45043a31e2806c843ffd > Reported-by: kernel test robot <lkp@xxxxxxxxx> > Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx> Seems okay to me. If it was something where performance might be signficiant I guess you could ifdef on CC_IS_CLANG, but for this it shouldn't matter. Acked-by: Nicholas Piggin <npiggin@xxxxxxxxx> Thanks, Nick > --- > arch/powerpc/kvm/book3s_hv_nested.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/powerpc/kvm/book3s_hv_nested.c b/arch/powerpc/kvm/book3s_hv_nested.c > index 60724f674421..1b3ff0af1264 100644 > --- a/arch/powerpc/kvm/book3s_hv_nested.c > +++ b/arch/powerpc/kvm/book3s_hv_nested.c > @@ -53,7 +53,8 @@ void kvmhv_save_hv_regs(struct kvm_vcpu *vcpu, struct hv_guest_state *hr) > hr->dawrx1 = vcpu->arch.dawrx1; > } > > -static void byteswap_pt_regs(struct pt_regs *regs) > +/* Use noinline_for_stack due to https://bugs.llvm.org/show_bug.cgi?id=49610 */ > +static noinline_for_stack void byteswap_pt_regs(struct pt_regs *regs) > { > unsigned long *addr = (unsigned long *) regs; > > > base-commit: 4a21192e2796c3338c4b0083b494a84a61311aaf > -- > 2.32.0.93.g670b81a890 > >
