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perf/amd/ibs: Support IBS_{FETCH|OP}_CTL2[Dis] to eliminate RMW race

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The existing IBS_{FETCH|OP}_CTL MSRs combine control and status bits
which leads to RMW race between HW and SW:

  HW                               SW
  ------------------------         ------------------------------
                                   config = rdmsr(IBS_OP_CTL);
                                   config &= ~EN;
  Set IBS_OP_CTL[Val] to 1
  trigger NMI
                                   wrmsr(IBS_OP_CTL, config);
                                   // Val is accidentally cleared

Future hardware adds a control-only MSR, IBS_{FETCH|OP}_CTL2, which
provides a second-level "disable" bit (Dis). IBS is now:

  Enabled:  IBS_{FETCH|OP}_CTL[En] = 1 && IBS_{FETCH|OP}_CTL2[Dis] = 0
  Disabled: IBS_{FETCH|OP}_CTL[En] = 0 || IBS_{FETCH|OP}_CTL2[Dis] = 1

The separate "Dis" bit lets software disable IBS without touching any
status fields, eliminating the hardware/software race.

Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260216042530.1546-4-ravi.bangoria@amd.com
This commit is contained in:
Ravi Bangoria 2026-02-16 04:25:26 +00:00 committed by Peter Zijlstra
parent e267b41781
commit efa5700ec0
1 changed files with 45 additions and 0 deletions
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45
arch/x86/events/amd/ibs.c
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@ -86,9 +86,11 @@ struct cpu_perf_ibs {
struct perf_ibs {
struct pmu pmu;
unsigned int msr;
unsigned int msr2;
u64 config_mask;
u64 cnt_mask;
u64 enable_mask;
u64 disable_mask;
u64 valid_mask;
u16 min_period;
u64 max_period;
@ -292,6 +294,8 @@ static int perf_ibs_init(struct perf_event *event)
return -ENOENT;
config = event->attr.config;
hwc->extra_reg.config = 0;
hwc->extra_reg.reg = 0;
if (event->pmu != &perf_ibs->pmu)
return -ENOENT;
@ -319,6 +323,11 @@ static int perf_ibs_init(struct perf_event *event)
if (perf_allow_kernel())
hwc->flags |= PERF_X86_EVENT_UNPRIVILEGED;
if (ibs_caps & IBS_CAPS_DIS) {
hwc->extra_reg.config &= ~perf_ibs->disable_mask;
hwc->extra_reg.reg = perf_ibs->msr2;
}
if (hwc->sample_period) {
if (config & perf_ibs->cnt_mask)
/* raw max_cnt may not be set */
@ -448,6 +457,9 @@ static inline void perf_ibs_enable_event(struct perf_ibs *perf_ibs,
wrmsrq(hwc->config_base, tmp & ~perf_ibs->enable_mask);
wrmsrq(hwc->config_base, tmp | perf_ibs->enable_mask);
if (hwc->extra_reg.reg)
wrmsrq(hwc->extra_reg.reg, hwc->extra_reg.config);
}
/*
@ -460,6 +472,11 @@ static inline void perf_ibs_enable_event(struct perf_ibs *perf_ibs,
static inline void perf_ibs_disable_event(struct perf_ibs *perf_ibs,
struct hw_perf_event *hwc, u64 config)
{
if (ibs_caps & IBS_CAPS_DIS) {
wrmsrq(hwc->extra_reg.reg, perf_ibs->disable_mask);
return;
}
config &= ~perf_ibs->cnt_mask;
if (boot_cpu_data.x86 == 0x10)
wrmsrq(hwc->config_base, config);
@ -812,6 +829,7 @@ static struct perf_ibs perf_ibs_fetch = {
.check_period = perf_ibs_check_period,
},
.msr = MSR_AMD64_IBSFETCHCTL,
.msr2 = MSR_AMD64_IBSFETCHCTL2,
.config_mask = IBS_FETCH_MAX_CNT | IBS_FETCH_RAND_EN,
.cnt_mask = IBS_FETCH_MAX_CNT,
.enable_mask = IBS_FETCH_ENABLE,
@ -837,6 +855,7 @@ static struct perf_ibs perf_ibs_op = {
.check_period = perf_ibs_check_period,
},
.msr = MSR_AMD64_IBSOPCTL,
.msr2 = MSR_AMD64_IBSOPCTL2,
.config_mask = IBS_OP_MAX_CNT,
.cnt_mask = IBS_OP_MAX_CNT | IBS_OP_CUR_CNT |
IBS_OP_CUR_CNT_RAND,
@ -1394,6 +1413,9 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
out:
if (!throttle) {
if (ibs_caps & IBS_CAPS_DIS)
wrmsrq(hwc->extra_reg.reg, perf_ibs->disable_mask);
if (perf_ibs == &perf_ibs_op) {
if (ibs_caps & IBS_CAPS_OPCNTEXT) {
new_config = period & IBS_OP_MAX_CNT_EXT_MASK;
@ -1465,6 +1487,9 @@ static __init int perf_ibs_fetch_init(void)
if (ibs_caps & IBS_CAPS_ZEN4)
perf_ibs_fetch.config_mask |= IBS_FETCH_L3MISSONLY;
if (ibs_caps & IBS_CAPS_DIS)
perf_ibs_fetch.disable_mask = IBS_FETCH_2_DIS;
perf_ibs_fetch.pmu.attr_groups = fetch_attr_groups;
perf_ibs_fetch.pmu.attr_update = fetch_attr_update;
@ -1486,6 +1511,9 @@ static __init int perf_ibs_op_init(void)
if (ibs_caps & IBS_CAPS_ZEN4)
perf_ibs_op.config_mask |= IBS_OP_L3MISSONLY;
if (ibs_caps & IBS_CAPS_DIS)
perf_ibs_op.disable_mask = IBS_OP_2_DIS;
perf_ibs_op.pmu.attr_groups = op_attr_groups;
perf_ibs_op.pmu.attr_update = op_attr_update;
@ -1732,6 +1760,23 @@ static void clear_APIC_ibs(void)
static int x86_pmu_amd_ibs_starting_cpu(unsigned int cpu)
{
setup_APIC_ibs();
if (ibs_caps & IBS_CAPS_DIS) {
/*
* IBS enable sequence:
* CTL[En] = 1;
* CTL2[Dis] = 0;
*
* IBS disable sequence:
* CTL2[Dis] = 1;
*
* Set CTL2[Dis] when CPU comes up. This is needed to make
* enable sequence effective.
*/
wrmsrq(MSR_AMD64_IBSFETCHCTL2, IBS_FETCH_2_DIS);
wrmsrq(MSR_AMD64_IBSOPCTL2, IBS_OP_2_DIS);
}
return 0;
}