x86,perf: Disable intel_bts when PTI

The intel_bts driver does not use the 'normal' BTS buffer which is exposed
through the cpu_entry_area but instead uses the memory allocated for the
perf AUX buffer.

This obviously comes apart when using PTI because then the kernel mapping;
which includes that AUX buffer memory; disappears. Fixing this requires to
expose a mapping which is visible in all context and that's not trivial.

As a quick fix disable this driver when PTI is enabled to prevent
malfunction.

Fixes: 385ce0ea4c07 ("x86/mm/pti: Add Kconfig")
Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Reported-by: Robert Święcki <robert@swiecki.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: greg@kroah.com
Cc: hughd@google.com
Cc: luto@amacapital.net
Cc: Vince Weaver <vince@deater.net>
Cc: torvalds@linux-foundation.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180114102713.GB6166@worktop.programming.kicks-ass.net

diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
index 141e07b0621689e745582599c009f4af1d053c6c..24ffa1e88cf948ecbe9839f6d956c69a12ecd4f0 100644 (file)
--- a/arch/x86/events/intel/bts.c
+++ b/arch/x86/events/intel/bts.c
@@ -582,6 +582,24 @@ static __init int bts_init(void)
        if (!boot_cpu_has(X86_FEATURE_DTES64) || !x86_pmu.bts)
                return -ENODEV;
 
+       if (boot_cpu_has(X86_FEATURE_PTI)) {
+               /*
+                * BTS hardware writes through a virtual memory map we must
+                * either use the kernel physical map, or the user mapping of
+                * the AUX buffer.
+                *
+                * However, since this driver supports per-CPU and per-task inherit
+                * we cannot use the user mapping since it will not be availble
+                * if we're not running the owning process.
+                *
+                * With PTI we can't use the kernal map either, because its not
+                * there when we run userspace.
+                *
+                * For now, disable this driver when using PTI.
+                */
+               return -ENODEV;
+       }
+
        bts_pmu.capabilities    = PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_ITRACE |
                                  PERF_PMU_CAP_EXCLUSIVE;
        bts_pmu.task_ctx_nr     = perf_sw_context;