Messages in this thread Patch in this message | | | Date | Tue, 30 Jan 2018 15:22:51 +0000 | From | Dave Martin <> | Subject | Re: [PATCH 15/16] arm64: Delay enabling hardware DBM feature |
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On Fri, Jan 26, 2018 at 04:05:24PM +0000, Suzuki K Poulose wrote: > On 26/01/18 14:41, Dave Martin wrote: > >On Tue, Jan 23, 2018 at 12:28:08PM +0000, Suzuki K Poulose wrote: > >>We enable hardware DBM bit in a capable CPU, very early in the > >>boot via __cpu_setup. This doesn't give us a flexibility of > >>optionally disable the feature, as the clearing the bit > >>is a bit costly as the TLB can cache the settings. Instead, > >>we delay enabling the feature until the CPU is brought up > >>into the kernel. We use the feature capability mechanism > >>to handle it. > >> > >>The hardware DBM is a non-conflicting feature. i.e, the kernel > >>can safely run with a mix of CPUs with some using the feature > >>and the others don't. So, it is safe for a late CPU to have > >>this capability and enable it, even if the active CPUs don't. > >> > >>To get this handled properly by the infrastructure, we > >>unconditionally set the capability and only enable it > >>on CPUs which really have the feature. Adds a new type > >>of feature to the capability infrastructure which > >>ignores the conflict in a late CPU. > >> > >>Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com> > >>--- > >> arch/arm64/include/asm/cpucaps.h | 3 ++- > >> arch/arm64/include/asm/cpufeature.h | 8 +++++++ > >> arch/arm64/kernel/cpufeature.c | 42 +++++++++++++++++++++++++++++++++++++ > >> arch/arm64/mm/proc.S | 5 +---- > >> 4 files changed, 53 insertions(+), 5 deletions(-) > >> > >>diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h > >>index bb263820de13..8df80cc828ac 100644 > >>--- a/arch/arm64/include/asm/cpucaps.h > >>+++ b/arch/arm64/include/asm/cpucaps.h > >>@@ -45,7 +45,8 @@ > >> #define ARM64_HARDEN_BRANCH_PREDICTOR 24 > >> #define ARM64_HARDEN_BP_POST_GUEST_EXIT 25 > >> #define ARM64_HAS_RAS_EXTN 26 > >>+#define ARM64_HW_DBM 27 > >>-#define ARM64_NCAPS 27 > >>+#define ARM64_NCAPS 28 > >> #endif /* __ASM_CPUCAPS_H */ > >>diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h > >>index 70712de687c7..243ec7c77c79 100644 > >>--- a/arch/arm64/include/asm/cpufeature.h > >>+++ b/arch/arm64/include/asm/cpufeature.h > >>@@ -126,6 +126,14 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0; > >> */ > >> #define ARM64_CPUCAP_STRICT_CPU_LOCAL_FEATURE \ > >> (ARM64_CPUCAP_SCOPE_LOCAL_CPU | ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS) > >>+/* > >>+ * CPU feature detected on each local CPU. It is safe for a late CPU to > >>+ * either have it or not. > >>+ */ > >>+#define ARM64_CPUCAP_WEAK_CPU_LOCAL_FEATURE \ > >>+ (ARM64_CPUCAP_SCOPE_LOCAL_CPU |\ > >>+ ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS |\ > >>+ ARM64_CPUCAP_LATE_CPU_SAFE_TO_HAVE) > > > >OK, so this is similar to my suggestion for HAS_NO_HW_PREFETCH (though > >that need not have the same answer -- I was speculating there). > > Yes, I was under the assumption that HAS_NO_HW_PREFETCH is treated as > a "Late CPU can't have the capability" type, hence the "STRICT_CPU_LOCAL", > as we can't apply work-arounds anymore for this CPU. However, since > we only suffer a performance impact, we could as well convert it to > a WEAK one.
(Note: I'm not very familiar with ThunderX. I may be assuming things I don't understand when I assert that only performance is affected here...)
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> >>diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c > >>index 2627a836e99d..8af755b8219d 100644 > >>--- a/arch/arm64/kernel/cpufeature.c > >>+++ b/arch/arm64/kernel/cpufeature.c > >>@@ -894,6 +894,35 @@ static int __init parse_kpti(char *str) > >> __setup("kpti=", parse_kpti); > >> #endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */ > >>+#ifdef CONFIG_ARM64_HW_AFDBM > >>+static bool has_hw_dbm(const struct arm64_cpu_capabilities *entry, int scope) > >>+{ > >>+ /* > >>+ * DBM is a non-conflicting feature. i.e, the kernel can safely run > >>+ * a mix of CPUs with and without the feature. So, we unconditionally > >>+ * enable the capability to allow any late CPU to use the feature. > >>+ * We only enable the control bits on the CPU, if it actually supports. > >>+ */ > >>+ return true; > >>+} > >>+ > >>+static inline void __cpu_enable_hw_dbm(void) > >>+{ > >>+ u64 tcr = read_sysreg(tcr_el1) | TCR_HD; > >>+ > >>+ write_sysreg(tcr, tcr_el1); > >>+ isb(); > > > >Do we need this isb? Do we care exactly when setting TCR_HD appears > >to take effect? > > Practically no, as it doesn't matter if we use it or not. But, since the > CPU is anyway booting, there is no harm in enforcing it to take effect.
Ok. Just wondering whether there was a requirement here that I wasn't understanding.
It would be worth a comment here explaining that the ISB is believed only to be improving determinism here, rather than being required for correctness.
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> >>@@ -1052,6 +1081,19 @@ static const struct arm64_cpu_capabilities arm64_features[] = { > >> .enable = cpu_clear_disr, > >> }, > >> #endif /* CONFIG_ARM64_RAS_EXTN */ > >>+#ifdef CONFIG_ARM64_HW_AFDBM > >>+ { > >>+ .desc = "Hardware pagetable Dirty Bit Management", > >>+ .type = ARM64_CPUCAP_WEAK_CPU_LOCAL_FEATURE, > >>+ .capability = ARM64_HW_DBM, > >>+ .sys_reg = SYS_ID_AA64MMFR1_EL1, > >>+ .sign = FTR_UNSIGNED, > >>+ .field_pos = ID_AA64MMFR1_HADBS_SHIFT, > >>+ .min_field_value = 2, > >>+ .matches = has_hw_dbm, > > > >Can't we use has_cpuid_feature here? Why do we need a fake .matches and > >then code the check manually in the enable mathod? > > We could, but then we need to add another *type*, where capabilities could > be enabled by a late CPU, where something is not already enabled by the boot-time > CPUs. i.e, if we boot a DBM capable CPU late, we won't be able to use the feature > on it, with the current setup. I didn't want to complicate the infrastructure > further just for this.
Fair enough. For now this seems like a unique case.
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Cheers ---Dave
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