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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id n5sm15609389pgf.35.2021.06.22.01.57.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Jun 2021 01:57:45 -0700 (PDT) Date: Tue, 22 Jun 2021 18:57:39 +1000 From: Nicholas Piggin Subject: Re: [PATCH] KVM: PPC: Book3S HV: Workaround high stack usage with clang To: Michael Ellerman , Nathan Chancellor , Paul Mackerras Cc: Arnd Bergmann , Benjamin Herrenschmidt , clang-built-linux@googlegroups.com, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kernel test robot , Nick Desaulniers References: <20210621182440.990242-1-nathan@kernel.org> In-Reply-To: <20210621182440.990242-1-nathan@kernel.org> MIME-Version: 1.0 Message-Id: <1624352117.tss3fnkwt4.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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: >=20 > 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=3D] > long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu) > ^ > 1 error generated. >=20 > 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: >=20 > 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=3D= ] > long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu) > ^ > 1 warning generated. >=20 > Link: https://github.com/ClangBuiltLinux/linux/issues/1292 > Link: https://bugs.llvm.org/show_bug.cgi?id=3D49610 > Link: https://lore.kernel.org/r/202104031853.vDT0Qjqj-lkp@intel.com/ > Link: https://gist.github.com/ba710e3703bf45043a31e2806c843ffd > Reported-by: kernel test robot > Signed-off-by: Nathan Chancellor Seems okay to me. If it was something where performance might be=20 signficiant I guess you could ifdef on CC_IS_CLANG, but for this it shouldn't matter. Acked-by: Nicholas Piggin Thanks, Nick > --- > arch/powerpc/kvm/book3s_hv_nested.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) >=20 > diff --git a/arch/powerpc/kvm/book3s_hv_nested.c b/arch/powerpc/kvm/book3= s_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 h= v_guest_state *hr) > hr->dawrx1 =3D vcpu->arch.dawrx1; > } > =20 > -static void byteswap_pt_regs(struct pt_regs *regs) > +/* Use noinline_for_stack due to https://bugs.llvm.org/show_bug.cgi?id= =3D49610 */ > +static noinline_for_stack void byteswap_pt_regs(struct pt_regs *regs) > { > unsigned long *addr =3D (unsigned long *) regs; > =20 >=20 > base-commit: 4a21192e2796c3338c4b0083b494a84a61311aaf > --=20 > 2.32.0.93.g670b81a890 >=20 >=20