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Mon, 30 Sep 2024 15:48:27 -0700 (PDT) Date: Mon, 30 Sep 2024 15:48:25 -0700 In-Reply-To: Mime-Version: 1.0 References: <20240911204158.2034295-1-seanjc@google.com> <20240911204158.2034295-4-seanjc@google.com> Message-ID: Subject: Re: [PATCH v2 03/13] KVM: selftests: Fudge around an apparent gcc bug in arm64's PMU test From: Sean Christopherson To: Marc Zyngier , Oliver Upton , Anup Patel , Paolo Bonzini , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, James Houghton Content-Type: text/plain; charset="us-ascii" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240930_154828_907899_2C780A42 X-CRM114-Status: GOOD ( 16.54 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Sep 30, 2024, Sean Christopherson wrote: > On Wed, Sep 11, 2024, Sean Christopherson wrote: > > Use u64_replace_bits() instead of u64p_replace_bits() to set PMCR.N in > > arm64's vPMU counter access test to fudge around what appears to be a gcc > > bug. With the recent change to have vcpu_get_reg() return a value in lieu > > of an out-param, some versions of gcc completely ignore the operation > > performed by set_pmcr_n(), i.e. ignore the output param. > > Filed a gcc bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116912 > > I'll report back if anything interesting comes out of that bug. Well, there goes several hours that I'll never get back. Selftests are compiled with -O2, which enables strict-aliasing optimizations, and "unsigned long" and "unsigned long long" technically don't alias despite being the same size on 64-bit builds, so the compiler is allowed to optimize away the load. *sigh* I'll replace this with a patch to disable strict-aliasing, which the kernel has done since forever (literally predates git). Grr. diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile index 48d32c5aa3eb..a6f92129bb02 100644 --- a/tools/testing/selftests/kvm/Makefile +++ b/tools/testing/selftests/kvm/Makefile @@ -235,10 +235,10 @@ CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \ -Wno-gnu-variable-sized-type-not-at-end -MD -MP -DCONFIG_64BIT \ -fno-builtin-memcmp -fno-builtin-memcpy \ -fno-builtin-memset -fno-builtin-strnlen \ - -fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \ - -I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \ - -I$(