[RFC PATCH 4/4] x86: ibs: Enable IBS profiling for memory accesses

Bharata B Rao
2025-03-06T05:48:57Z

Enable IBS memory access data collection for user memory
accesses by programming the required MSRs. The profiling
is turned ON only for user mode execution and turned OFF
for kernel mode execution. Profiling is explicitly disabled
for NMI handler too.

TODOs:

- IBS sampling rate is kept fixed for now.
- Arch/vendor separation/isolation of the code needs relook.

Signed-off-by: Bharata B Rao <bharata@amd.com>
---
arch/x86/include/asm/entry-common.h | 3 +++
arch/x86/include/asm/hardirq.h | 2 ++
arch/x86/include/asm/ibs.h | 2 ++
arch/x86/mm/ibs.c | 32 +++++++++++++++++++++++++++++
4 files changed, 39 insertions(+)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 77d20555e04d..8127111c6ad3 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -9,10 +9,12 @@
#include <asm/io_bitmap.h>
#include <asm/fpu/api.h>
#include <asm/fred.h>
+#include <asm/ibs.h>

/* Check that the stack and regs on entry from user mode are sane. */
static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs)
{
+ hw_access_profiling_stop();
if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) {
/*
* Make sure that the entry code gave us a sensible EFLAGS
@@ -98,6 +100,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
static __always_inline void arch_exit_to_user_mode(void)
{
amd_clear_divider();
+ hw_access_profiling_start();
}
#define arch_exit_to_user_mode arch_exit_to_user_mode

diff --git a/arch/x86/include/asm/hardirq.h b/arch/x86/include/asm/hardirq.h
index 6ffa8b75f4cd..b928fbbcf3e5 100644
--- a/arch/x86/include/asm/hardirq.h
+++ b/arch/x86/include/asm/hardirq.h
@@ -91,4 +91,6 @@ static __always_inline bool kvm_get_cpu_l1tf_flush_l1d(void)
static __always_inline void kvm_set_cpu_l1tf_flush_l1d(void) { }
#endif /* IS_ENABLED(CONFIG_KVM_INTEL) */

+#define arch_nmi_enter() hw_access_profiling_stop()
+#define arch_nmi_exit() hw_access_profiling_start()
#endif /* _ASM_X86_HARDIRQ_H */
diff --git a/arch/x86/include/asm/ibs.h b/arch/x86/include/asm/ibs.h
index b5a4f2ca6330..6b480958534e 100644
--- a/arch/x86/include/asm/ibs.h
+++ b/arch/x86/include/asm/ibs.h
@@ -2,6 +2,8 @@
#ifndef _ASM_X86_IBS_H
#define _ASM_X86_IBS_H

+void hw_access_profiling_start(void);
+void hw_access_profiling_stop(void);
extern bool arch_hw_access_profiling;

#endif /* _ASM_X86_IBS_H */
diff --git a/arch/x86/mm/ibs.c b/arch/x86/mm/ibs.c
index 5c966050ad86..961d0c67ca50 100644
--- a/arch/x86/mm/ibs.c
+++ b/arch/x86/mm/ibs.c
@@ -15,6 +15,7 @@ bool arch_hw_access_profiling;
static u64 ibs_config __read_mostly;
static u32 ibs_caps;

+#define IBS_SAMPLE_PERIOD 10000
#define IBS_NR_SAMPLES 50

/*
@@ -99,6 +100,36 @@ static void ibs_irq_handler(struct irq_work *i)
schedule_work_on(smp_processor_id(), &ibs_work);
}

+void hw_access_profiling_stop(void)
+{
+ u64 ops_ctl;
+
+ if (!arch_hw_access_profiling)
+ return;
+
+ rdmsrl(MSR_AMD64_IBSOPCTL, ops_ctl);
+ wrmsrl(MSR_AMD64_IBSOPCTL, ops_ctl & ~IBS_OP_ENABLE);
+}
+
+void hw_access_profiling_start(void)
+{
+ u64 config = 0;
+ unsigned int period = IBS_SAMPLE_PERIOD;
+
+ if (!arch_hw_access_profiling)
+ return;
+
+ /* Disable IBS for kernel thread */
+ if (!current->mm)
+ goto out;
+
+ config = (period >> 4) & IBS_OP_MAX_CNT;
+ config |= (period & IBS_OP_MAX_CNT_EXT_MASK);
+ config |= ibs_config;
+out:
+ wrmsrl(MSR_AMD64_IBSOPCTL, config);
+}
+
/*
* IBS NMI handler: Process the memory access info reported by IBS.
*
@@ -305,6 +336,7 @@ static int __init ibs_access_profiling_init(void)
x86_amd_ibs_access_profile_startup,
x86_amd_ibs_access_profile_teardown);

+ arch_hw_access_profiling = true;
pr_info("IBS setup for memory access profiling\n");
return 0;
}
--
2.34.1