Re: [PATCH] arm64: Add ARM64_HAS_LSE2 CPU capability

tiantao (H)
2024-09-06T10:59:56Z

在 2024/9/6 17:44, Mark Rutland 写道:
> On Fri, Sep 06, 2024 at 05:08:12PM +0800, Tian Tao wrote:
>> When FEAT_LSE2 is implemented and Bit 6 of sctlr_elx is nAA, the
>> full name of the Not-aligned access. nAA bit has two values:
>> 0b0 Unaligned accesses by the specified instructions generate an
>> Alignment fault.
>> 0b1 Unaligned accesses by the specified instructions do not generate
>> an Alignment fault.
>>
>> this patch sets the nAA bit to 1,The following instructions will not
>> generate an Alignment fault if all bytes being accessed are not within
>> a single 16-byte quantity:
>> • LDAPR, LDAPRH, LDAPUR, LDAPURH, LDAPURSH, LDAPURSW, LDAR, LDARH,LDLAR,
>> LDLARH.
>> • STLLR, STLLRH, STLR, STLRH, STLUR, and STLURH
>>
>> Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
> What is going to depend on this? Nothing in the kernel depends on being
> able to make unaligned accesses with these instructions, and (since you
> haven't added a HWCAP), userspace has no idea that these accesses won't
> generate an alignment fault.
>
> Mark.

I've come across a situation where the simplified code is as follows:

long  address = (long) mmap(NULL,1024*1024*2,PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS,-1,0);

long new_address = address + 9;

long *p = (long*) new_address;
long v = -1;

__atomic_store(p, &v, __ATOMIC_RELEASE);

coredump occurs after executing __atomic_store, but the user code can't
be changed,

so I'm trying to enable NAA to solve this problem.

>> ---
>> arch/arm64/Kconfig | 10 ++++++++++
>> arch/arm64/include/asm/sysreg.h | 1 +
>> arch/arm64/kernel/cpufeature.c | 18 ++++++++++++++++++
>> arch/arm64/tools/cpucaps | 1 +
>> 4 files changed, 30 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 77d7ef0b16c2..7afe73ebcd79 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -2023,6 +2023,16 @@ config ARM64_TLB_RANGE
>> The feature introduces new assembly instructions, and they were
>> support when binutils >= 2.30.
>>
>> +config ARM64_LSE2_NAA
>> + bool "Enable support for not-aligned access"
>> + depends on AS_HAS_ARMV8_4
>> + help
>> + LSE2 is an extension to the original LSE (Large System Extensions) feature,
>> + introduced in ARMv8.4.
>> +
>> + Enable this feature will not generate an Alignment fault if all bytes being
>> + accessed are not within a single 16-byte quantity.
>> +
>> endmenu # "ARMv8.4 architectural features"
>>
>> menu "ARMv8.5 architectural features"
>> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
>> index 8cced8aa75a9..42e3a1959aa8 100644
>> --- a/arch/arm64/include/asm/sysreg.h
>> +++ b/arch/arm64/include/asm/sysreg.h
>> @@ -854,6 +854,7 @@
>> #define SCTLR_ELx_ENDB (BIT(13))
>> #define SCTLR_ELx_I (BIT(12))
>> #define SCTLR_ELx_EOS (BIT(11))
>> +#define SCTLR_ELx_nAA (BIT(6))
>> #define SCTLR_ELx_SA (BIT(3))
>> #define SCTLR_ELx_C (BIT(2))
>> #define SCTLR_ELx_A (BIT(1))
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 646ecd3069fd..558869a7c7f0 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -2299,6 +2299,14 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
>> }
>> #endif /* CONFIG_ARM64_MTE */
>>
>> +#ifdef CONFIG_ARM64_LSE2_NAA
>> +static void cpu_enable_lse2(const struct arm64_cpu_capabilities *__unused)
>> +{
>> + sysreg_clear_set(sctlr_el2, SCTLR_ELx_nAA, SCTLR_ELx_nAA);
>> + isb();
>> +}
>> +#endif
>> +
>> static void user_feature_fixup(void)
>> {
>> if (cpus_have_cap(ARM64_WORKAROUND_2658417)) {
>> @@ -2427,6 +2435,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>> ARM64_CPUID_FIELDS(ID_AA64ISAR0_EL1, ATOMIC, IMP)
>> },
>> #endif /* CONFIG_ARM64_LSE_ATOMICS */
>> +#ifdef CONFIG_ARM64_LSE2_NAA
>> + {
>> + .desc = "Support for not-aligned access",
>> + .capability = ARM64_HAS_LSE2,
>> + .type = ARM64_CPUCAP_SYSTEM_FEATURE,
>> + .matches = has_cpuid_feature,
>> + .cpu_enable = cpu_enable_lse2,
>> + ARM64_CPUID_FIELDS(ID_AA64MMFR2_EL1, AT, IMP)
>> + },
>> +#endif
>> {
>> .desc = "Virtualization Host Extensions",
>> .capability = ARM64_HAS_VIRT_HOST_EXTN,
>> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
>> index ac3429d892b9..0c7c0a293574 100644
>> --- a/arch/arm64/tools/cpucaps
>> +++ b/arch/arm64/tools/cpucaps
>> @@ -41,6 +41,7 @@ HAS_HCX
>> HAS_LDAPR
>> HAS_LPA2
>> HAS_LSE_ATOMICS
>> +HAS_LSE2
>> HAS_MOPS
>> HAS_NESTED_VIRT
>> HAS_PAN
>> --
>> 2.33.0
>>
>>
> .
>