Re: [PATCH] arm64: Add ARM64_HAS_LSE2 CPU capability

tiantao (H)
2024-09-06T11:32:35Z

在 2024/9/6 19:09, Mark Rutland 写道:
> On Fri, Sep 06, 2024 at 06:58:19PM +0800, tiantao (H) wrote:
>> 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,
> Where is that code and why can't it be changed? That code has never
> worked and would always have generated a coredump, and even with this
> patch it cannot work on hardware without LSE2.
>
This code works fine on x86 platforms and does not coredump.
>> so I'm trying to enable NAA to solve this problem.
> AFAICT that's making a kernel change to paper over a userspace bug.
> As-is I don't think that's a good justification for changing nAA.
>
> Mark.
armv8.4 support nAA as a feature that should be enabled in the kernel?
>>>> ---
>>>> 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
>>>>
>>>>
>>> .
>>>
> .
>