On Fri, Feb 28, 2025 at 06:45:38PM -0800, Yang Shi wrote:

On 2/28/25 5:29 PM, Yang Shi wrote:

quoted

quoted

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 940343beb3d4..baae6d458996 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_ENABLE_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..108ef3fbbc00 100644

--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h

@@ -866,6 +866,11 @@ 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);
+}

Hi Miko,

I added AmpereOne mdir on top of this patch. I can see BBML2 feature is
detected via dmesg. But system_supports_bbml2_noabort() returns false.
The warning in the below debug patch is triggered:

diff --git a/arch/arm64/kernel/cpufeature.c

b/arch/arm64/kernel/cpufeature.c
index faa9094d97dd..a70829ae2bd0 100644

--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -3814,6 +3814,9 @@ void __init setup_system_features(void)

 {
        setup_system_capabilities();

+       if (!system_supports_bbml2_noabort())
+               WARN_ON_ONCE(1);
+
        kpti_install_ng_mappings();

        sve_setup();

I thought it may be too early. But it seems other system features work
well, for example, MPAM. I didn't figure out why. It is weird.

I just figured out the problem It is because the wrong kconfig name is used
in cpucaps.h. The code is:

+    case ARM64_HAS_BBML2_NOABORT:
+        return IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT);

But the kconfig name actually is:

+config ARM64_ENABLE_BBML2_NOABORT

IMHO, the "ENABLE" in kconfig name sounds unnecessary.

Thanks,
Yang

Hi Yang,

Thank you for the review, and apologies for the slight delay.

Thanks again for the spot, I agree that `ENABLE` is probably redundant
(and clearly, caused an issue here). Will remove this. Please let me
know if there are any other issues with rebasing your patches on top of
mine.

-- 
Kind regards,
Mikołaj Lenczewski