Re: [PATCH] KVM: PPC: Book3S HV: Workaround high stack usage with clang
From: Nicholas Piggin <npiggin@gmail.com>
Date: 2021-06-22 08:57:50
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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@intel.com/ (local)
Link: https://gist.github.com/ba710e3703bf45043a31e2806c843ffd
Reported-by: kernel test robot <redacted>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>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@gmail.com> Thanks, Nick
quoted hunk ↗ jump to hunk
--- 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