linux-kernel - Re: [PATCH v2 2/2] x86/cpufeatures: Enumerate new AVX512 BFLOAT16 instructions

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Message-ID: <20190620103720.GB28032@zn.tnic>
Date:   Thu, 20 Jun 2019 12:37:20 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 2/2] x86/cpufeatures: Enumerate new AVX512 BFLOAT16
 instructions

On Wed, Jun 19, 2019 at 02:34:04PM -0700, Fenghua Yu wrote:
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index efb114298cfb..4910cb421b82 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -847,6 +847,9 @@ void get_cpu_cap(struct cpuinfo_x86 *c)
>  		c->x86_capability[CPUID_7_0_EBX] = ebx;
>  		c->x86_capability[CPUID_7_ECX] = ecx;
>  		c->x86_capability[CPUID_7_EDX] = edx;
> +
> +		cpuid_count(0x00000007, 1, &eax, &ebx, &ecx, &edx);
> +		c->x86_capability[CPUID_7_1_EAX] = eax;
>  	}

You need to test the sub-leaf index validity here before accessing
subleaf 1:

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 4910cb421b82..dad20bc891d5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -848,8 +848,11 @@ void get_cpu_cap(struct cpuinfo_x86 *c)
 		c->x86_capability[CPUID_7_ECX] = ecx;
 		c->x86_capability[CPUID_7_EDX] = edx;
 
-		cpuid_count(0x00000007, 1, &eax, &ebx, &ecx, &edx);
-		c->x86_capability[CPUID_7_1_EAX] = eax;
+		/* Check valid sub-leaf index before accessing it */
+		if (eax >= 1) {
+			cpuid_count(0x00000007, 1, &eax, &ebx, &ecx, &edx);
+			c->x86_capability[CPUID_7_1_EAX] = eax;
+		}
 	}
 
 	/* Extended state features: level 0x0000000d */

-- 
Regards/Gruss,
    Boris.

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