linux-kernel - Re: [RFC] perf/x86/intel: Disable check

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Date:   Mon, 24 Jun 2019 00:40:31 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...el.com>, Jiri Olsa <jolsa@...nel.org>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Tom Vaden <tom.vaden@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Alok Kataria <akataria@...are.com>
Subject: Re: [RFC] perf/x86/intel: Disable check_msr for real hw

On Fri, Jun 21, 2019 at 10:48:25AM -0700, Andi Kleen wrote:
> On Fri, Jun 14, 2019 at 01:28:53PM +0200, Jiri Olsa wrote:
> > hi,
> > the HPE server can do POST tracing and have enabled LBR
> > tracing during the boot, which makes check_msr fail falsly.
> > 
> > It looks like check_msr code was added only to check on guests
> > MSR access, would it be then ok to disable check_msr for real
> > hardware? (as in patch below)
> > 
> > We could also check if LBR tracing is enabled and make
> > appropriate checks, but this change is simpler ;-)
> > 
> > ideas? thanks,
> > jirka
> 
> Sorry for the late comment. I see this patch has been merged now.
> 
> Unfortunately I don't think it's a good idea. The problem 
> is that the hypervisor flags are only set for a few hypervisors
> that Linux knows about. But in practice there are many more
> Hypervisors around that will not cause these flags to be set.
> But these are still likely to miss MSRs.
> 
> The other hypervisors are relatively obscure, but eventually
> someone will hit problems.

any idea if there's any other flag/way we could use to detect those?

adding few virtualization folks to the loop
and attaching the original patch

thanks,
jirka


---
Tom Vaden reported false failure of check_msr function, because
some servers can do POST tracing and enable LBR tracing during
the boot.

Kan confirmed that check_msr patch was to fix a bug report in
guest, so it's ok to disable it for real HW.

Cc: Kan Liang <kan.liang@...el.com>
Reported-by: Tom Vaden <tom.vaden@....com>
Signed-off-by: Jiri Olsa <jolsa@...nel.org>
---
 arch/x86/events/intel/core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 71001f005bfe..1194ae7e1992 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -20,6 +20,7 @@
 #include <asm/intel-family.h>
 #include <asm/apic.h>
 #include <asm/cpu_device_id.h>
+#include <asm/hypervisor.h>
 
 #include "../perf_event.h"
 
@@ -4050,6 +4051,13 @@ static bool check_msr(unsigned long msr, u64 mask)
 {
 	u64 val_old, val_new, val_tmp;
 
+	/*
+	 * Disable the check for real HW, so we don't
+	 * mess up with potentionaly enabled regs.
+	 */
+	if (hypervisor_is_type(X86_HYPER_NATIVE))
+		return true;
+
 	/*
 	 * Read the current value, change it and read it back to see if it
 	 * matches, this is needed to detect certain hardware emulators
-- 
2.21.0