On 13/03/2025 10:41, Mikołaj Lenczewski wrote:
> The Break-Before-Make cpu feature supports multiple levels (levels 0-2),
> and this commit adds a dedicated BBML2 cpufeature to test against
> support for, as well as a kernel commandline parameter to optionally
> disable BBML2 altogether.
>
> This is a system feature as we might have a big.LITTLE architecture
> where some cores support BBML2 and some don't, but we want all cores to
> be available and BBM to default to level 0 (as opposed to having cores
> without BBML2 not coming online).
>
> To support BBML2 in as wide a range of contexts as we can, we want not
> only the architectural guarantees that BBML2 makes, but additionally
> want BBML2 to not create TLB conflict aborts. Not causing aborts avoids
> us having to prove that no recursive faults can be induced in any path
> that uses BBML2, allowing its use for arbitrary kernel mappings.
> Support detection of such CPUs.
>
> Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@xxxxxxx>
I have 2 nits below, but with those resolved:
Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
> .../admin-guide/kernel-parameters.txt | 3 +
> arch/arm64/Kconfig | 11 +++
> arch/arm64/include/asm/cpucaps.h | 2 +
> arch/arm64/include/asm/cpufeature.h | 6 ++
> arch/arm64/kernel/cpufeature.c | 76 +++++++++++++++++++
> arch/arm64/kernel/pi/idreg-override.c | 2 +
> arch/arm64/tools/cpucaps | 1 +
> 7 files changed, 101 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index fb8752b42ec8..3e4cc917a07e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -453,6 +453,9 @@
> arm64.no32bit_el0 [ARM64] Unconditionally disable the execution of
> 32 bit applications.
>
> + arm64.nobbml2 [ARM64] Unconditionally disable Break-Before-Make Level
> + 2 support
> +
> arm64.nobti [ARM64] Unconditionally disable Branch Target
> Identification support
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 940343beb3d4..49deda2b22ae 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2057,6 +2057,17 @@ config ARM64_TLB_RANGE
> The feature introduces new assembly instructions, and they were
> support when binutils >= 2.30.
>
> +config ARM64_BBML2_NOABORT
> + bool "Enable support for Break-Before-Make Level 2 detection and usage"
> + default y
> + help
> + FEAT_BBM provides detection of support levels for break-before-make
> + sequences. If BBM level 2 is supported, some TLB maintenance requirements
> + can be relaxed to improve performance. We additonally require the
> + property that the implementation cannot ever raise TLB Conflict Aborts.
> + Selecting N causes the kernel to fallback to BBM level 0 behaviour
> + even if the system supports BBM level 2.
> +
> endmenu # "ARMv8.4 architectural features"
>
> menu "ARMv8.5 architectural features"
> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> index 0b5ca6e0eb09..2d6db33d4e45 100644
> --- a/arch/arm64/include/asm/cpucaps.h
> +++ b/arch/arm64/include/asm/cpucaps.h
> @@ -23,6 +23,8 @@ cpucap_is_possible(const unsigned int cap)
> return IS_ENABLED(CONFIG_ARM64_PAN);
> case ARM64_HAS_EPAN:
> return IS_ENABLED(CONFIG_ARM64_EPAN);
> + case ARM64_HAS_BBML2_NOABORT:
> + return IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT);
> case ARM64_SVE:
> return IS_ENABLED(CONFIG_ARM64_SVE);
> case ARM64_SME:
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index e0e4478f5fb5..7f5b220dacde 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -18,6 +18,7 @@
> #define ARM64_SW_FEATURE_OVERRIDE_NOKASLR 0
> #define ARM64_SW_FEATURE_OVERRIDE_HVHE 4
> #define ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF 8
> +#define ARM64_SW_FEATURE_OVERRIDE_NOBBML2 12
>
> #ifndef __ASSEMBLY__
>
> @@ -866,6 +867,11 @@ static __always_inline bool system_supports_mpam_hcr(void)
> return alternative_has_cap_unlikely(ARM64_MPAM_HCR);
> }
>
> +static inline bool system_supports_bbml2_noabort(void)
> +{
> + return alternative_has_cap_unlikely(ARM64_HAS_BBML2_NOABORT);
> +}
> +
> int do_emulate_mrs(struct pt_regs *regs, u32 sys_reg, u32 rt);
> bool try_emulate_mrs(struct pt_regs *regs, u32 isn);
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index d561cf3b8ac7..b936e0805161 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2176,6 +2176,76 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
> return arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_HVHE);
> }
>
> +static inline bool bbml2_possible(void)
> +{
> + return !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
If you're going to keep this helper, I think it really needs to be:
return IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT) &&
!arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
Then you would simplify the caller to remove it's own
IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT) check.
But personally I would remove the helper and just fold the test into
has_bbml2_noabort().
Thanks,
Ryan
> +}
> +
> +static bool cpu_has_bbml2_noabort(unsigned int cpu_midr)
> +{
> + /* We want to allow usage of bbml2 in as wide a range of kernel contexts
> + * as possible. This list is therefore an allow-list of known-good
> + * implementations that both support bbml2 and additionally, fulfill the
> + * extra constraint of never generating TLB conflict aborts when using
> + * the relaxed bbml2 semantics (such aborts make use of bbml2 in certain
> + * kernel contexts difficult to prove safe against recursive aborts).
> + *
> + * Note that implementations can only be considered "known-good" if their
> + * implementors attest to the fact that the implementation never raises
> + * TLBI conflict aborts for bbml2 mapping granularity changes.
> + */
> + static const struct midr_range supports_bbml2_noabort_list[] = {
> + MIDR_REV_RANGE(MIDR_CORTEX_X4, 0, 3, 0xf),
> + MIDR_REV_RANGE(MIDR_NEOVERSE_V3, 0, 2, 0xf),
> + {}
> + };
> +
> + return is_midr_in_range_list(cpu_midr, supports_bbml2_noabort_list);
> +}
> +
> +static inline unsigned int __cpu_read_midr(int cpu)
nit: why the double underscrore prefix?
> +{
> + WARN_ON_ONCE(!cpu_online(cpu));
> +
> + return per_cpu(cpu_data, cpu).reg_midr;
> +}
> +
> +static bool has_bbml2_noabort(const struct arm64_cpu_capabilities *caps, int scope)
> +{
> + if (!IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT))
> + return false;
> +
> + if (!bbml2_possible())
> + return false;
> +
> + if (scope & SCOPE_SYSTEM) {
> + int cpu;
> +
> + /* We are a boot CPU, and must verify that all enumerated boot
> + * CPUs have MIDR values within our allowlist. Otherwise, we do
> + * not allow the BBML2 feature to avoid potential faults when
> + * the insufficient CPUs access memory regions using BBML2
> + * semantics.
> + */
> + for_each_online_cpu(cpu) {
> + if (!cpu_has_bbml2_noabort(__cpu_read_midr(cpu)))
> + return false;
> + }
> +
> + return true;
> + } else if (scope & SCOPE_LOCAL_CPU) {
> + /* We are a hot-plugged CPU, so only need to check our MIDR.
> + * If we have the correct MIDR, but the kernel booted on an
> + * insufficient CPU, we will not use BBML2 (this is safe). If
> + * we have an incorrect MIDR, but the kernel booted on a
> + * sufficient CPU, we will not bring up this CPU.
> + */
> + return cpu_has_bbml2_noabort(read_cpuid_id());
> + }
> +
> + return false;
> +}
> +
> #ifdef CONFIG_ARM64_PAN
> static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
> {
> @@ -2926,6 +2996,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
> .matches = has_cpuid_feature,
> ARM64_CPUID_FIELDS(ID_AA64MMFR2_EL1, EVT, IMP)
> },
> + {
> + .desc = "BBM Level 2 without conflict abort",
> + .capability = ARM64_HAS_BBML2_NOABORT,
> + .type = ARM64_CPUCAP_SYSTEM_FEATURE,
> + .matches = has_bbml2_noabort,
> + },
> {
> .desc = "52-bit Virtual Addressing for KVM (LPA2)",
> .capability = ARM64_HAS_LPA2,
> diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
> index c6b185b885f7..9728faa10390 100644
> --- a/arch/arm64/kernel/pi/idreg-override.c
> +++ b/arch/arm64/kernel/pi/idreg-override.c
> @@ -209,6 +209,7 @@ static const struct ftr_set_desc sw_features __prel64_initconst = {
> FIELD("nokaslr", ARM64_SW_FEATURE_OVERRIDE_NOKASLR, NULL),
> FIELD("hvhe", ARM64_SW_FEATURE_OVERRIDE_HVHE, hvhe_filter),
> FIELD("rodataoff", ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF, NULL),
> + FIELD("nobbml2", ARM64_SW_FEATURE_OVERRIDE_NOBBML2, NULL),
> {}
> },
> };
> @@ -246,6 +247,7 @@ static const struct {
> { "rodata=off", "arm64_sw.rodataoff=1" },
> { "arm64.nolva", "id_aa64mmfr2.varange=0" },
> { "arm64.no32bit_el0", "id_aa64pfr0.el0=1" },
> + { "arm64.nobbml2", "arm64_sw.nobbml2=1" },
> };
>
> static int __init parse_hexdigit(const char *p, u64 *v)
> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
> index 1e65f2fb45bd..b03a375e5507 100644
> --- a/arch/arm64/tools/cpucaps
> +++ b/arch/arm64/tools/cpucaps
> @@ -14,6 +14,7 @@ HAS_ADDRESS_AUTH_ARCH_QARMA5
> HAS_ADDRESS_AUTH_IMP_DEF
> HAS_AMU_EXTN
> HAS_ARMv8_4_TTL
> +HAS_BBML2_NOABORT
> HAS_CACHE_DIC
> HAS_CACHE_IDC
> HAS_CNP
