Re: [PATCH 8/8] arm64: Work around systems with mismatched cache line sizes

From: Will Deacon
Date: Mon Aug 22 2016 - 09:02:30 EST

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On Thu, Aug 18, 2016 at 02:10:32PM +0100, Suzuki K Poulose wrote:
> Systems with differing CPU i-cache/d-cache line sizes can cause
> problems with the cache management by software when the execution
> is migrated from one to another. Usually, the application reads
> the cache size on a CPU and then uses that length to perform cache
> operations. However, if it gets migrated to another CPU with a smaller
> cache line size, things could go completely wrong. To prevent such
> cases, always use the smallest cache line size among the CPUs. The
> kernel CPU feature infrastructure already keeps track of the safe
> value for all CPUID registers including CTR. This patch works around
> the problem by :
>
> For kernel, dynamically patch the kernel to read the cache size
> from the system wide copy of CTR_EL0.

Is it only CTR that is mismatched in practice, or do we need to worry
about DCZID_EL0 too?

> For applications, trap read accesses to CTR_EL0 (by clearing the SCTLR.UCT)
> and emulate the mrs instruction to return the system wide safe value
> of CTR_EL0.
>
> For faster access (i.e, avoiding to lookup the system wide value of CTR_EL0
> via read_system_reg), we keep track of the pointer to table entry for
> CTR_EL0 in the CPU feature infrastructure.
>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Andre Przywara <andre.przywara@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> ---
> arch/arm64/include/asm/assembler.h | 25 +++++++++++++++++++++++--
> arch/arm64/include/asm/cpufeature.h | 4 +++-
> arch/arm64/include/asm/esr.h | 8 ++++++++
> arch/arm64/include/asm/sysreg.h | 1 +
> arch/arm64/kernel/asm-offsets.c | 2 ++
> arch/arm64/kernel/cpu_errata.c | 22 ++++++++++++++++++++++
> arch/arm64/kernel/cpufeature.c | 9 +++++++++
> arch/arm64/kernel/traps.c | 14 ++++++++++++++
> 8 files changed, 82 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index a4bb3f5..66bd268 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -216,6 +216,21 @@ lr .req x30 // link register
> .macro mmid, rd, rn
> ldr \rd, [\rn, #MM_CONTEXT_ID]
> .endm
> +/*
> + * read_ctr - read CTR_EL0. If the system has mismatched
> + * cache line sizes, provide the system wide safe value.
> + */
> + .macro read_ctr, reg
> +alternative_if_not ARM64_MISMATCHED_CACHE_LINE_SIZE
> + mrs \reg, ctr_el0 // read CTR
> + nop
> + nop
> +alternative_else
> + ldr_l \reg, sys_ctr_ftr // Read system wide safe CTR value
> + ldr \reg, [\reg, #ARM64_FTR_SYSVAL] // from sys_ctr_ftr->sys_val
> +alternative_endif
> + .endm
> +
>
> /*
> * raw_dcache_line_size - get the minimum D-cache line size on this CPU
> @@ -232,7 +247,10 @@ lr .req x30 // link register
> * dcache_line_size - get the safe D-cache line size across all CPUs
> */
> .macro dcache_line_size, reg, tmp
> - raw_dcache_line_size \reg, \tmp
> + read_ctr \tmp
> + ubfm \tmp, \tmp, #16, #19 // cache line size encoding
> + mov \reg, #4 // bytes per word
> + lsl \reg, \reg, \tmp // actual cache line size
> .endm
>
> /*
> @@ -250,7 +268,10 @@ lr .req x30 // link register
> * icache_line_size - get the safe I-cache line size across all CPUs
> */
> .macro icache_line_size, reg, tmp
> - raw_icache_line_size \reg, \tmp
> + read_ctr \tmp
> + and \tmp, \tmp, #0xf // cache line size encoding
> + mov \reg, #4 // bytes per word
> + lsl \reg, \reg, \tmp // actual cache line size
> .endm
>
> /*
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 692b8d3..e99f2af 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -37,8 +37,9 @@
> #define ARM64_WORKAROUND_CAVIUM_27456 12
> #define ARM64_HAS_32BIT_EL0 13
> #define ARM64_HYP_OFFSET_LOW 14
> +#define ARM64_MISMATCHED_CACHE_LINE_SIZE 15
>
> -#define ARM64_NCAPS 15
> +#define ARM64_NCAPS 16
>
> #ifndef __ASSEMBLY__
>
> @@ -109,6 +110,7 @@ struct arm64_cpu_capabilities {
> };
>
> extern DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
> +extern struct arm64_ftr_reg *sys_ctr_ftr;
>
> bool this_cpu_has_cap(unsigned int cap);
>
> diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
> index 2a8f6c3..51aea89 100644
> --- a/arch/arm64/include/asm/esr.h
> +++ b/arch/arm64/include/asm/esr.h
> @@ -139,6 +139,9 @@
> ((Op2) << ESR_ELx_SYS64_ISS_Op2_SHIFT) | \
> ((CRn) << ESR_ELx_SYS64_ISS_CRn_SHIFT) | \
> ((CRm) << ESR_ELx_SYS64_ISS_CRm_SHIFT))
> +
> +#define ESR_ELx_SYS64_ISS_SYS_OP_MASK (ESR_ELx_SYS64_ISS_SYS_MASK | \
> + ESR_ELx_SYS64_ISS_DIR_MASK)
> /*
> * User space cache operations have the following sysreg encoding
> * in System instructions.
> @@ -157,6 +160,11 @@
> #define ESR_ELx_SYS64_ISS_U_CACHE_OP_VAL \
> (ESR_ELx_SYS64_ISS_SYS_VAL(1, 3, 1, 7, 0) | \
> ESR_ELx_SYS64_ISS_DIR_WRITE)
> +
> +#define ESR_ELx_SYS64_ISS_SYS_CTR ESR_ELx_SYS64_ISS_SYS_VAL(3, 3, 1, 0, 0)
> +#define ESR_ELx_SYS64_ISS_SYS_CTR_READ (ESR_ELx_SYS64_ISS_SYS_CTR | \
> + ESR_ELx_SYS64_ISS_DIR_READ)
> +
> #ifndef __ASSEMBLY__
> #include <asm/types.h>
>
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index cc06794..4f26ad5 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -100,6 +100,7 @@
> /* SCTLR_EL1 specific flags. */
> #define SCTLR_EL1_UCI (1 << 26)
> #define SCTLR_EL1_SPAN (1 << 23)
> +#define SCTLR_EL1_UCT (1 << 15)
> #define SCTLR_EL1_SED (1 << 8)
> #define SCTLR_EL1_CP15BEN (1 << 5)
>
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 05070b7..4a2f0f0 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -23,6 +23,7 @@
> #include <linux/dma-mapping.h>
> #include <linux/kvm_host.h>
> #include <linux/suspend.h>
> +#include <asm/cpufeature.h>
> #include <asm/thread_info.h>
> #include <asm/memory.h>
> #include <asm/smp_plat.h>
> @@ -145,5 +146,6 @@ int main(void)
> DEFINE(HIBERN_PBE_ORIG, offsetof(struct pbe, orig_address));
> DEFINE(HIBERN_PBE_ADDR, offsetof(struct pbe, address));
> DEFINE(HIBERN_PBE_NEXT, offsetof(struct pbe, next));
> + DEFINE(ARM64_FTR_SYSVAL, offsetof(struct arm64_ftr_reg, sys_val));
> return 0;
> }
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index 5836b3d..621fa7f 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -30,6 +30,21 @@ is_affected_midr_range(const struct arm64_cpu_capabilities *entry, int scope)
> entry->midr_range_max);
> }
>
> +static bool
> +has_mismatched_cache_line_size(const struct arm64_cpu_capabilities *entry,
> + int scope)
> +{
> + WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
> + return (read_cpuid_cachetype() & sys_ctr_ftr->strict_mask) !=
> + (sys_ctr_ftr->sys_val & sys_ctr_ftr->strict_mask);
> +}
> +
> +static void cpu_enable_trap_ctr_access(void *__unused)
> +{
> + /* Clear SCTLR_EL1.UCT */
> + config_sctlr_el1(SCTLR_EL1_UCT, 0);
> +}
> +
> #define MIDR_RANGE(model, min, max) \
> .def_scope = SCOPE_LOCAL_CPU, \
> .matches = is_affected_midr_range, \
> @@ -108,6 +123,13 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
> },
> #endif
> {
> + .desc = "Mismatched cache line size",
> + .capability = ARM64_MISMATCHED_CACHE_LINE_SIZE,
> + .matches = has_mismatched_cache_line_size,
> + .def_scope = SCOPE_LOCAL_CPU,
> + .enable = cpu_enable_trap_ctr_access,
> + },
> + {
> }
> };
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index fcf87ca..c1db313 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -46,6 +46,13 @@ unsigned int compat_elf_hwcap2 __read_mostly;
>
> DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
>
> +/*
> + * If we have mismatched cache line sizes, we need to emulate access to
> + * CTRL_EL0. To avoid a lookup everytime (via read_system_reg()), cache
> + * the table entry for CTR_EL0.
> + */
> +struct arm64_ftr_reg *sys_ctr_ftr;
> +
> #define __ARM64_FTR_BITS(SIGNED, STRICT, TYPE, SHIFT, WIDTH, SAFE_VAL) \
> { \
> .sign = SIGNED, \
> @@ -461,6 +468,8 @@ void __init init_cpu_features(struct cpuinfo_arm64 *info)
> init_cpu_ftr_reg(SYS_MVFR2_EL1, info->reg_mvfr2);
> }
>
> + /* Keep track of the CTR feature register */
> + sys_ctr_ftr = get_arm64_ftr_reg(SYS_CTR_EL0);
> }
>
> static void update_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 new)
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index 93c5287..db2d6cb 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -480,6 +480,14 @@ static void user_cache_maint_handler(unsigned int esr, struct pt_regs *regs)
> regs->pc += 4;
> }
>
> +static void ctr_read_handler(unsigned int esr, struct pt_regs *regs)
> +{
> + int rt = (esr & ESR_ELx_SYS64_ISS_RT_MASK) >> ESR_ELx_SYS64_ISS_RT_SHIFT;
> +
> + regs->regs[rt] = sys_ctr_ftr->sys_val;
> + regs->pc += 4;
> +}

Whilst this is correct, I wonder if there's any advantage in reporting a
*larger* size to userspace and avoid incurring additional trap overhead?

Any idea what sort of size typical JITs are using?

Will

Next message: David Hildenbrand: "Re: [PATCH 2/4] KVM-S390: Use memdup_user() rather than duplicating its implementation"
Previous message: Jens Wiklander: "[PATCH v11 3/4] tee: add OP-TEE driver"
In reply to: Suzuki K Poulose: "[PATCH 8/8] arm64: Work around systems with mismatched cache line sizes"
Next in thread: Suzuki K Poulose: "[PATCH 7/8] arm64: Refactor sysinstr exception handling"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]