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Mon, 15 Jun 2026 05:59:10 -0700 (PDT) Message-ID: <7d9eef31-2193-4593-8976-7c7659843bf2@gmail.com> Date: Mon, 15 Jun 2026 20:59:06 +0800 Precedence: bulk X-Mailing-List: asahi@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] arm64: errata: Handle Apple WFI State Loss Content-Language: en-MW To: Yureka Lilian , Catalin Marinas , Will Deacon Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, asahi@lists.linux.dev, Sasha Finkelstein References: <20260615-wfi-erratum-v2-1-59a73467f70d@cyberchaos.dev> From: Nick Chan In-Reply-To: <20260615-wfi-erratum-v2-1-59a73467f70d@cyberchaos.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Yureka Lilian 於 2026/6/15 晚上8:21 寫道: > Apple Silicon CPUs can lose register state in WFI, leading to crashes > in the idle loop early in the boot process. > This applies to any previous Apple Silicon CPUs too, but is worked > around by configuring the WFI mode in SYS_IMP_APL_CYC_OVRD sysreg > during m1n1's chickens setup. > This workaround no longer exists since M4. > > Add a workaround capability for replacing wfi and wfit with nop, and > an erratum to enable it on the affected CPUs if the workaround using the > sysreg is not already applied. Leave the decision whether the sysreg > workaround can be used up to the earlier parts of the boot chain which > already configure the Apple Silicon chicken bits. > > This alternative has to be applied in early boot, since otherwise some > cores might enter the idle loop before apply_alternatives_all() is run. > > Reviewed-by: Sasha Finkelstein > Signed-off-by: Yureka Lilian > --- > Changes since v1: > Restricted the erratum to EL2 only, since in EL1 we'd expect the > hypervisor to trap WFI and handle the erratum. > > Tested on M4 and M4 Pro (which now sometimes nondeterministically > crash later during boot). > Successfully booted on M3 Max with the SYS_IMP_APL_CYC_OVRD > workaround disabled in the bootloader, as well as A18 Pro (which, > like M4 / M4 Pro, doesn't have SYS_IMP_APL_CYC_OVRD). > > There is probably a better place for the SYS_IMP_APL_CYC_OVRD > defines, which I currently put in the middle of cpu_errata.c, but I > wouldn't know where. > --- > arch/arm64/Kconfig | 12 ++++++++++++ > arch/arm64/include/asm/barrier.h | 19 ++++++++++++++++--- > arch/arm64/kernel/cpu_errata.c | 21 +++++++++++++++++++++ > arch/arm64/tools/cpucaps | 1 + > 4 files changed, 50 insertions(+), 3 deletions(-) > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index b3afe0688919..8c8ff069856f 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -453,6 +453,18 @@ config AMPERE_ERRATUM_AC04_CPU_23 > > If unsure, say Y. > > +config APPLE_ERRATUM_WFI_STATE > + bool "Apple Silicon: WFI loses state" > + default y > + help > + This option adds an alternative code sequence to work around some > + Apple Silicon CPUs losing register state during wfi and wfit > + instructions. > + > + As a workaround, the wfi and wfit instructions are replaced with nop > + operations via the alternative framework if an affected CPU is > + detected. > + > config ARM64_WORKAROUND_CLEAN_CACHE > bool > > diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h > index 9495c4441a46..f72eddc7c434 100644 > --- a/arch/arm64/include/asm/barrier.h > +++ b/arch/arm64/include/asm/barrier.h > @@ -20,9 +20,22 @@ > #define wfe() asm volatile("wfe" : : : "memory") > #define wfet(val) asm volatile("msr s0_3_c1_c0_0, %0" \ > : : "r" (val) : "memory") > -#define wfi() asm volatile("wfi" : : : "memory") > -#define wfit(val) asm volatile("msr s0_3_c1_c0_1, %0" \ > - : : "r" (val) : "memory") > +#define wfi() \ > + do { \ > + asm volatile( \ > + ALTERNATIVE("wfi", \ > + "nop", \ > + ARM64_WORKAROUND_WFI_STATE) \ > + : : : "memory"); \ > + } while (0) > +#define wfit(val) \ > + do { \ > + asm volatile( \ > + ALTERNATIVE("msr s0_3_c1_c0_1, %0", \ > + "nop", \ > + ARM64_WORKAROUND_WFI_STATE) \ > + : : "r" (val) : "memory"); \ > + } while (0) > > #define isb() asm volatile("isb" : : : "memory") > #define dmb(opt) asm volatile("dmb " #opt : : : "memory") > diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c > index 1995e1198648..8c9a194eddc4 100644 > --- a/arch/arm64/kernel/cpu_errata.c > +++ b/arch/arm64/kernel/cpu_errata.c > @@ -309,6 +309,19 @@ static void cpu_enable_impdef_pmuv3_traps(const struct arm64_cpu_capabilities *_ > sysreg_clear_set_s(SYS_HACR_EL2, 0, BIT(56)); > } > > +#ifdef CONFIG_APPLE_ERRATUM_WFI_STATE > +static bool has_apple_erratum_wfi_state(const struct arm64_cpu_capabilities *entry, int scope) > +{ > +#define SYS_IMP_APL_CYC_OVRD sys_reg(3, 5, 15, 5, 0) > +#define CYC_OVRD_WFI_MODE_MASK GENMASK(26, 24) > + if (read_cpuid_implementor() != ARM_CPU_IMP_APPLE) > + return false; > + if ((read_sysreg(CurrentEL) >> 2) != 2) > + return false; Nested vitrualization exists, and may be supported by KVM or macOS HVF. Additionally, presumably the workaround should be applied under m1n1 hypervisor. The solution is less clear. A reliable way to detect bare metal or "almost bare metal" is checking for "apple,arm-platform" using of_machine_is_compatible(), but that involves performing a non-CPU check inside a CPU errata function, so that do not feel right to me. Best Regards, Nick Chan > + return FIELD_GET(CYC_OVRD_WFI_MODE_MASK, read_sysreg_s(SYS_IMP_APL_CYC_OVRD)) != 2; > +} > +#endif > + > #ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI > static const struct arm64_cpu_capabilities arm64_repeat_tlbi_list[] = { > #ifdef CONFIG_QCOM_FALKOR_ERRATUM_1009 > @@ -1009,6 +1022,14 @@ const struct arm64_cpu_capabilities arm64_errata[] = { > .matches = has_impdef_pmuv3, > .cpu_enable = cpu_enable_impdef_pmuv3_traps, > }, > +#ifdef CONFIG_APPLE_ERRATUM_WFI_STATE > + { > + .desc = "Apple WFI loses state", > + .capability = ARM64_WORKAROUND_WFI_STATE, > + .type = ARM64_CPUCAP_SCOPE_BOOT_CPU | ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU, > + .matches = has_apple_erratum_wfi_state, > + }, > +#endif > { > } > }; > diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps > index 9b85a84f6fd4..bbf8c15d79b0 100644 > --- a/arch/arm64/tools/cpucaps > +++ b/arch/arm64/tools/cpucaps > @@ -128,3 +128,4 @@ WORKAROUND_REPEAT_TLBI > WORKAROUND_SPECULATIVE_AT > WORKAROUND_SPECULATIVE_SSBS > WORKAROUND_SPECULATIVE_UNPRIV_LOAD > +WORKAROUND_WFI_STATE > > --- > base-commit: c425609d6ac4012c8bbf01ec2e10e801b1923a7b > change-id: 20260614-wfi-erratum-7a9f305f601f > > Best regards, > -- > Yureka Lilian > >