linux-kernel - [PATCH] x86/cpu: Avoid endless loop to get the number of cache leaves

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Date:   Mon, 5 Nov 2018 20:53:45 +0800
From:   Pu Wen <puwen@...on.cn>
To:     <bp@...en8.de>, <tglx@...utronix.de>, <mingo@...hat.com>,
        <hpa@...or.com>, <x86@...nel.org>, <linux-kernel@...r.kernel.org>
CC:     Pu Wen <puwen@...on.cn>
Subject: [PATCH] x86/cpu: Avoid endless loop to get the number of cache leaves

To get the number of cache leaves on AMD or Hygon platform, it should
get the value of cpuid leaf 0x8000001d. But on certain broken platform
such as a not fullly implemented virtual platform(Xen, for example),
the value of the cpuid leaf will nerver be CTYPE_NULL, so the kernel
will run into an endless loop.

To fix this problem, add a new enum type CTYPE_MAX to limit the maximum
cpuid accessing.

Signed-off-by: Pu Wen <puwen@...on.cn>
---
 arch/x86/kernel/cpu/cacheinfo.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/cacheinfo.c b/arch/x86/kernel/cpu/cacheinfo.c
index dc1b934..7bd167f 100644
--- a/arch/x86/kernel/cpu/cacheinfo.c
+++ b/arch/x86/kernel/cpu/cacheinfo.c
@@ -121,7 +121,8 @@ enum _cache_type {
 	CTYPE_NULL = 0,
 	CTYPE_DATA = 1,
 	CTYPE_INST = 2,
-	CTYPE_UNIFIED = 3
+	CTYPE_UNIFIED = 3,
+	CTYPE_MAX = 4
 };
 
 union _cpuid4_leaf_eax {
@@ -640,7 +641,7 @@ static int find_num_cache_leaves(struct cpuinfo_x86 *c)
 		/* Do cpuid(op) loop to find out num_cache_leaves */
 		cpuid_count(op, i, &eax, &ebx, &ecx, &edx);
 		cache_eax.full = eax;
-	} while (cache_eax.split.type != CTYPE_NULL);
+	} while (cache_eax.split.type != CTYPE_NULL && i != CTYPE_MAX);
 	return i;
 }
 
-- 
2.7.4