[PATCH v2] x86/amd: Disable RDSEED on AMD Zen5 because of an error.

[Gregory Price <gourry@gourry.net>]

Under unknown conditions, Zen5 chips running rdseed can produce
(val=0,CF=1) over 10% of the time (when rdseed is successful).
CF=1 indicates success, while val=0 is typically only produced
when rdseed fails (CF=0).

This suggests there is a bug which causes rdseed to silently fail.

This was reproduced reliably by launching 2-threads per available
core, 1-thread per for hamming on RDSEED, and 1-thread per core
collectively eating and hammering on ~90% of memory.

This was observed on more than 1 Zen5 model, so it should be disabled
for all of Zen5 until/unless a comprehensive blacklist can be built.

Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price <gourry@gourry.net>
---
 arch/x86/kernel/cpu/amd.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 5398db4dedb4..1af30518d3e7 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1037,6 +1037,10 @@ static void init_amd_zen4(struct cpuinfo_x86 *c)
 
 static void init_amd_zen5(struct cpuinfo_x86 *c)
 {
+	/* Disable RDSEED on AMD Turin because of an error. */
+	clear_cpu_cap(c, X86_FEATURE_RDSEED);
+	msr_clear_bit(MSR_AMD64_CPUID_FN_7, 18);
+	pr_emerg("RDSEED is not reliable on this platform; disabling.\n");
 }
 
 static void init_amd(struct cpuinfo_x86 *c)
-- 
2.51.0