linux-kernel - Re: [PATCH 200/208] x86/fpu/xstate: Don't assume the first zero xfeatures zero bit means the end

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Date:	Tue, 05 May 2015 13:10:46 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
CC:	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 200/208] x86/fpu/xstate: Don't assume the first zero xfeatures
 zero bit means the end

On 05/05/2015 10:58 AM, Ingo Molnar wrote:
> -	do {
> +	for (leaf = 2; leaf < xfeatures_nr; leaf++) {
>  		cpuid_count(XSTATE_CPUID, leaf, &eax, &ebx, &ecx, &edx);
>  
> -		if (eax == 0)
> -			break;
> -
>  		xstate_offsets[leaf] = ebx;
>  		xstate_sizes[leaf] = eax;
>  
> +		printk(KERN_INFO "x86/fpu: xstate_offset[%d]: %04x, xstate_sizes[%d]: %04x\n", leaf, ebx, leaf, eax);
>  		leaf++;
> -	} while (1);
> +	}

We're going to have to revisit this.  There's a new SDM out that makes
this incorrect.  It says:

"If support for state component i is limited to XSAVES and XRSTORS, EBX
returns 0."
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