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SubjectRe: [PATCH] x86/cpu: Avoid endless loop to get the number of cache leaves
On Mon, Nov 05, 2018 at 08:53:45PM +0800, Pu Wen wrote:
> 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@hygon.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);

i is an int and CTYPE_MAX is enum _cache_type. Huh?

This works by chance because CTYPE_MAX is 4 and the termination CPUID
leaf is the 4th too.

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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