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@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 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@xxxxxxx
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
--
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