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

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Date:   Thu, 15 Nov 2018 18:21:55 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Pu Wen <puwen@...on.cn>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [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@...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);

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.