2015-05-19 00:00:44

by Thomas Gleixner

Subject: [patch 1/6] x86, perf, cqm: Document PQR MSR abuse

The cqm code acts like it owns the PQR MSR completely. That's not true
because only the lower 10 bits are used for CQM. The upper 32bits are
used for CLass Of Service ID (closid). Document the abuse. Will be
fixed in a later patch.

Signed-off-by: Thomas Gleixner <[email protected]>
---
arch/x86/kernel/cpu/perf_event_intel_cqm.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)

Index: linux/arch/x86/kernel/cpu/perf_event_intel_cqm.c
===================================================================
--- linux.orig/arch/x86/kernel/cpu/perf_event_intel_cqm.c
+++ linux/arch/x86/kernel/cpu/perf_event_intel_cqm.c
@@ -978,7 +978,12 @@ static void intel_cqm_event_start(struct
WARN_ON_ONCE(state->rmid);

state->rmid = rmid;
- wrmsrl(MSR_IA32_PQR_ASSOC, state->rmid);
+ /*
+ * This is actually wrong, as the upper 32 bit MSR contain the
+ * closid which is used for configuring the Cache Allocation
+ * Technology component.
+ */
+ wrmsr(MSR_IA32_PQR_ASSOC, rmid, 0);

raw_spin_unlock_irqrestore(&state->lock, flags);
}
@@ -998,7 +1003,13 @@ static void intel_cqm_event_stop(struct

if (!--state->cnt) {
state->rmid = 0;
- wrmsrl(MSR_IA32_PQR_ASSOC, 0);
+ /*
+ * This is actually wrong, as the upper 32 bit of the
+ * MSR contain the closid which is used for
+ * configuring the Cache Allocation Technology
+ * component.
+ */
+ wrmsr(MSR_IA32_PQR_ASSOC, 0, 0);
} else {
WARN_ON_ONCE(!state->rmid);
}

2015-05-19 11:54:00

by Matt Fleming

Subject: Re: [patch 1/6] x86, perf, cqm: Document PQR MSR abuse

On Tue, 19 May, at 12:00:50AM, Thomas Gleixner wrote:
> The cqm code acts like it owns the PQR MSR completely. That's not true
> because only the lower 10 bits are used for CQM. The upper 32bits are
> used for CLass Of Service ID (closid). Document the abuse. Will be
> fixed in a later patch.
>
> Signed-off-by: Thomas Gleixner <[email protected]>
> ---
> arch/x86/kernel/cpu/perf_event_intel_cqm.c | 15 +++++++++++++--
> 1 file changed, 13 insertions(+), 2 deletions(-)

Acked-by: Matt Fleming <[email protected]>

--
Matt Fleming, Intel Open Source Technology Center