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

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Message-ID: <1544237214-18766-1-git-send-email-puwen@hygon.cn>
Date:   Sat, 8 Dec 2018 10:46:54 +0800
From:   Pu Wen <puwen@...on.cn>
To:     <bp@...en8.de>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        Pu Wen <puwen@...on.cn>
Subject: Re: [PATCH] x86/cpu: Avoid endless loop to get the number of cache leaves

Sorry for the late reply :)

On 2018/11/16 1:22, Borislav Petkov wrote:
>> @@ -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?

How about define CTYPE_MAX like this:
#define CTYPE_MAX  4

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

It will return CTYPE_NULL when accessing the 4th CPUID leaf in most of
the cases, but in certain case it will not. So I think it's better to
restrict the maximum CPUID access times to 4 for kernel robustness.

-- 
Regards,
Pu Wen