linux-kernel - [tip: perf/urgent] perf/x86: Avoid touching LBR

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Date:   Wed, 19 May 2021 08:02:52 -0000
From:   "tip-bot2 for Like Xu" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Like Xu <like.xu@...ux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: perf/urgent] perf/x86: Avoid touching LBR_TOS MSR for Arch LBR

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     3317c26a4b413b41364f2c4b83c778c6aba1576d
Gitweb:        https://git.kernel.org/tip/3317c26a4b413b41364f2c4b83c778c6aba1576d
Author:        Like Xu <like.xu@...ux.intel.com>
AuthorDate:    Fri, 30 Apr 2021 13:22:46 +08:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 18 May 2021 12:53:47 +02:00

perf/x86: Avoid touching LBR_TOS MSR for Arch LBR

The Architecture LBR does not have MSR_LBR_TOS (0x000001c9).
In a guest that should support Architecture LBR, check_msr()
will be a non-related check for the architecture MSR 0x0
(IA32_P5_MC_ADDR) that is also not supported by KVM.

The failure will cause x86_pmu.lbr_nr = 0, thereby preventing
the initialization of the guest Arch LBR. Fix it by avoiding
this extraneous check in intel_pmu_init() for Arch LBR.

Fixes: 47125db27e47 ("perf/x86/intel/lbr: Support Architectural LBR")
Signed-off-by: Like Xu <like.xu@...ux.intel.com>
[peterz: simpler still]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20210430052247.3079672-1-like.xu@linux.intel.com
---
 arch/x86/events/intel/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 2521d03..e288922 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -6253,7 +6253,7 @@ __init int intel_pmu_init(void)
 	 * Check all LBT MSR here.
 	 * Disable LBR access if any LBR MSRs can not be accessed.
 	 */
-	if (x86_pmu.lbr_nr && !check_msr(x86_pmu.lbr_tos, 0x3UL))
+	if (x86_pmu.lbr_tos && !check_msr(x86_pmu.lbr_tos, 0x3UL))
 		x86_pmu.lbr_nr = 0;
 	for (i = 0; i < x86_pmu.lbr_nr; i++) {
 		if (!(check_msr(x86_pmu.lbr_from + i, 0xffffUL) &&