linux-kernel - [PATCH v3 1/2] x86/AMD: Fix cpu_llc

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Date:   Tue, 1 Nov 2016 11:51:02 -0500
From:   Yazen Ghannam <Yazen.Ghannam@....com>
To:     <linux-kernel@...r.kernel.org>
CC:     <x86@...nel.org>, <bp@...e.de>,
        Yazen Ghannam <Yazen.Ghannam@....com>, <stable@...r.kernel.org>
Subject: [PATCH v3 1/2] x86/AMD: Fix cpu_llc_id for AMD Fam17h systems

The current Fam17h cpu_llc_id (Last Level Cache ID) derivation has an
underflow bug when extracting the socket_id value. The socket_id value
starts from 0, so subtracting 1 will result in an underflow. This breaks
scheduling topology later on since the cpu_llc_id will be incorrect.

The APICID decoding is fixed, in register, for bits 3 and above, which give
the core complex, node and socket IDs. The LLC is at the core complex level
so we can find a unique cpu_llc_id by right shifting the APICID by 3
because then the least significant bit will be the Core Complex ID.

We can fix the underflow bug and simplify the code by replacing the
current cpu_llc_id derivation with a right shift.

Signed-off-by: Yazen Ghannam <Yazen.Ghannam@....com>
Cc: <stable@...r.kernel.org> # v4.4..
Fixes: 3849e91f571d ("x86/AMD: Fix last level cache topology for AMD Fam17h systems")
---
Link:
http://lkml.kernel.org/r/1477669918-56261-1-git-send-email-Yazen.Ghannam@amd.com

v2->v3:
* Fixup commit message based on comments.

 arch/x86/kernel/cpu/amd.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index b81fe2d..1e81a37 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -347,7 +347,6 @@ static void amd_detect_cmp(struct cpuinfo_x86 *c)
 #ifdef CONFIG_SMP
 	unsigned bits;
 	int cpu = smp_processor_id();
-	unsigned int socket_id, core_complex_id;

 	bits = c->x86_coreid_bits;
 	/* Low order bits define the core id (index of core in socket) */
@@ -365,10 +364,7 @@ static void amd_detect_cmp(struct cpuinfo_x86 *c)
 	 if (c->x86 != 0x17 || !cpuid_edx(0x80000006))
 		return;

-	socket_id	= (c->apicid >> bits) - 1;
-	core_complex_id	= (c->apicid & ((1 << bits) - 1)) >> 3;
-
-	per_cpu(cpu_llc_id, cpu) = (socket_id << 3) | core_complex_id;
+	per_cpu(cpu_llc_id, cpu) = c->apicid >> 3;
 #endif
 }

-- 
1.9.1