linux-kernel - Re: [PATCH v7 03/10] x86, mpx: add macro cpu_has

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Date:	Tue, 22 Jul 2014 09:18:52 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Qiaowei Ren <qiaowei.ren@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v7 03/10] x86, mpx: add macro cpu_has_mpx

On 07/20/2014 10:38 PM, Qiaowei Ren wrote:
> +#ifdef CONFIG_X86_INTEL_MPX
> +#define cpu_has_mpx boot_cpu_has(X86_FEATURE_MPX)
> +#else
> +#define cpu_has_mpx 0
> +#endif /* CONFIG_X86_INTEL_MPX */

Is this enough checking?  Looking at the extension reference, it says:

> 9.3.3
> Enabling of Intel MPX States
> An OS can enable Intel MPX states to support software operation using bounds registers with the following steps:
> • Verify the processor supports XSAVE/XRSTOR/XSETBV/XGETBV instructions and XCR0 by checking
> CPUID.1.ECX.XSAVE[bit 26]=1.

That, I assume the xsave code is already doing.

> • Verify the processor supports both Intel MPX states by checking CPUID.(EAX=0x0D, ECX=0):EAX[4:3] is 11b.

I see these bits _attempting_ to get set in pcntxt_mask via XCNTXT_MASK.
 But, I don't see us ever actually checking that they _do_ get set.  For
instance, we do this for:

>         if ((pcntxt_mask & XSTATE_FPSSE) != XSTATE_FPSSE) {
>                 pr_err("FP/SSE not shown under xsave features 0x%llx\n",
>                        pcntxt_mask);
>                 BUG();
>         }

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