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If memory bandwidth associated with a CLOSID is aggressively > throttled, and a task with that CLOSID moves into kernel mode, the kernel > operations are also aggressively throttled. This can stall forward progress > and eventually degrade overall system performance. > > AMD hardware supports a feature Privilege Level Zero Association (PLZA), > which allows the CPU's CLOSID association to be changed during the > transition from user mode to kernel mode. This allows the kernel to run > using a different CLOSID than user space, which can improve system > performance in certain scenarios. > > The feature is detected via CPUID_Fn80000020_EBX_x00 [Bit 9]: > Privilege Level Zero Association (PLZA). > > The PLZA feature details are documented in [1] available from [2]. > > [1] AMD64 Zen6 Platform Quality of Service (PQOS) Extensions: > Publication # 69193 Revision: 1.00, Issue Date: March 2026 > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 # [2] > Signed-off-by: Babu Moger > --- > v4: Split the patch into 2. This patch only handles x86 changes. > Re-wrote the changelog along the ABMC changes. > > v3: Code did not change. Patch order changed. > Added documentation link. > > v2: Rebased on top of the latest tip. > --- > arch/x86/include/asm/cpufeatures.h | 1 + > arch/x86/kernel/cpu/scattered.c | 1 + > 2 files changed, 2 insertions(+) > > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h > index 1b4a48bff18f..bf6fc71f87fa 100644 > --- a/arch/x86/include/asm/cpufeatures.h > +++ b/arch/x86/include/asm/cpufeatures.h > @@ -517,6 +517,7 @@ > * and purposes if CLEAR_CPU_BUF_VM is set). > */ > #define X86_FEATURE_X2AVIC_EXT (21*32+20) /* AMD SVM x2AVIC support for 4k vCPUs */ > +#define X86_FEATURE_PLZA (21*32+21) /* Privilege Level Zero Association */ > > /* > * BUG word(s) > diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c > index 937129ce6a96..1fa4d5aef17e 100644 > --- a/arch/x86/kernel/cpu/scattered.c > +++ b/arch/x86/kernel/cpu/scattered.c > @@ -61,6 +61,7 @@ static const struct cpuid_bit cpuid_bits[] = { > { X86_FEATURE_BMEC, CPUID_EBX, 3, 0x80000020, 0 }, > { X86_FEATURE_ABMC, CPUID_EBX, 5, 0x80000020, 0 }, > { X86_FEATURE_SDCIAE, CPUID_EBX, 6, 0x80000020, 0 }, > + { X86_FEATURE_PLZA, CPUID_EBX, 9, 0x80000020, 0 }, > { X86_FEATURE_TSA_SQ_NO, CPUID_ECX, 1, 0x80000021, 0 }, > { X86_FEATURE_TSA_L1_NO, CPUID_ECX, 2, 0x80000021, 0 }, > { X86_FEATURE_AMD_WORKLOAD_CLASS, CPUID_EAX, 22, 0x80000021, 0 }, Sashiko reported: --- Patch [1]: [RESEND PATCH v4 01/15] x86/resctrl: Support Privilege Level Zero Association (PLZA) --- commit 0f2aa093b81d7a2e0dc203ce3466562f708898df Author: Babu Moger x86/resctrl: Support Privilege Level Zero Association (PLZA) This commit adds support for AMD's Privilege Level Zero Association (PLZA) feature. It detects the feature via CPUID_Fn80000020_EBX_x00 [Bit 9] and adds the corresponding feature flag to the x86 cpufeatures array. Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 # [2] > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h > index 1b4a48bff18f7..bf6fc71f87fab 100644 > --- a/arch/x86/include/asm/cpufeatures.h > +++ b/arch/x86/include/asm/cpufeatures.h > @@ -517,6 +517,7 @@ > * and purposes if CLEAR_CPU_BUF_VM is set). > */ > #define X86_FEATURE_X2AVIC_EXT (21*32+20) /* AMD SVM x2AVIC support for 4k vCPUs */ > +#define X86_FEATURE_PLZA (21*32+21) /* Privilege Level Zero Association */ Should this addition also be mirrored in the tools directory copies of cpufeatures.h and kcpuid.csv? If tools/arch/x86/include/asm/cpufeatures.h is not updated to match, tools like perf can generate build warnings during check-headers.sh verification. Also, without updating tools/arch/x86/kcpuid/cpuid.csv, the kcpuid tool will be unaware of the new PLZA feature bit when users query CPU capabilities. I thought tools/arch/x86/include/asm/cpufeatures.h would be synced after the changes to arch/x86/include/asm/cpufeatures.h are merged. Has that process changed? If the expectation is to update tools/arch/x86/include/asm/cpufeatures.h as part of this patch, I can add those changes as well. Thanks, Babu