linux-kernel - Re: [v8, 4/5] x86/xsave: Make XSAVE check the base CPUID features before enabling

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Date:   Sat, 11 Nov 2017 15:15:21 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     x86@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [v8, 4/5] x86/xsave: Make XSAVE check the base CPUID features
 before enabling

On Thu, Oct 05, 2017 at 02:52:55PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Before enabling XSAVE, not only check the XSAVE specific CPUID bits,
> but also the base CPUID features of the respective XSAVE feature.
> This allows to disable individual XSAVE states using the existing
> clearcpuid= option, which can be useful for performance testing
> and debugging, and also in general avoids inconsistencies.
> 
> v2:
> Add curly brackets (Thomas Gleixner)
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

this patch makes qemu spit out the following warning, possibly because
it doesn't support all expected CPUID features for the various CPUs.

WARNING: CPU: 0 PID: 0 at arch/x86/kernel/fpu/xstate.c:614
	fpu__init_system_xstate+0x403/0x721

with associated traceback.

I understand that this is a qemu problem, but does the traceback really
add value ?

Guenter

> ---
>  arch/x86/kernel/fpu/xstate.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index f1d5476c9022..924bd895b5ee 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -15,6 +15,7 @@
>  #include <asm/fpu/xstate.h>
>  
>  #include <asm/tlbflush.h>
> +#include <asm/cpufeature.h>
>  
>  /*
>   * Although we spell it out in here, the Processor Trace
> @@ -36,6 +37,19 @@ static const char *xfeature_names[] =
>  	"unknown xstate feature"	,
>  };
>  
> +static short xsave_cpuid_features[] = {
> +	X86_FEATURE_FPU,
> +	X86_FEATURE_XMM,
> +	X86_FEATURE_AVX,
> +	X86_FEATURE_MPX,
> +	X86_FEATURE_MPX,
> +	X86_FEATURE_AVX512F,
> +	X86_FEATURE_AVX512F,
> +	X86_FEATURE_AVX512F,
> +	X86_FEATURE_INTEL_PT,
> +	X86_FEATURE_PKU,
> +};
> +
>  /*
>   * Mask of xstate features supported by the CPU and the kernel:
>   */
> @@ -726,6 +740,7 @@ void __init fpu__init_system_xstate(void)
>  	unsigned int eax, ebx, ecx, edx;
>  	static int on_boot_cpu __initdata = 1;
>  	int err;
> +	int i;
>  
>  	WARN_ON_FPU(!on_boot_cpu);
>  	on_boot_cpu = 0;
> @@ -759,6 +774,14 @@ void __init fpu__init_system_xstate(void)
>  		goto out_disable;
>  	}
>  
> +	/*
> +	 * Clear XSAVE features that are disabled in the normal CPUID.
> +	 */
> +	for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
> +		if (!boot_cpu_has(xsave_cpuid_features[i]))
> +			xfeatures_mask &= ~BIT(i);
> +	}
> +
>  	xfeatures_mask &= fpu__get_supported_xfeatures_mask();
>  
>  	/* Enable xstate instructions to be able to continue with initialization: */