linux-kernel - Re: [PATCH v7 1/6] x86/fpu/xstate: Differentiate default features for host and guest FPUs

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Message-ID: <aC84WRnMsIcozhbf@intel.com>
Date: Thu, 22 May 2025 22:44:41 +0800
From: Chao Gao <chao.gao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <x86@...nel.org>, <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<tglx@...utronix.de>, <dave.hansen@...el.com>, <pbonzini@...hat.com>,
	<peterz@...radead.org>, <rick.p.edgecombe@...el.com>,
	<weijiang.yang@...el.com>, <john.allen@....com>, <bp@...en8.de>,
	<chang.seok.bae@...el.com>, <xin3.li@...el.com>, Ingo Molnar
	<mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter
 Anvin" <hpa@...or.com>, Samuel Holland <samuel.holland@...ive.com>, "Mitchell
 Levy" <levymitchell0@...il.com>, Kees Cook <kees@...nel.org>, "Stanislav
 Spassov" <stanspas@...zon.de>, Eric Biggers <ebiggers@...gle.com>, "Nikolay
 Borisov" <nik.borisov@...e.com>, Oleg Nesterov <oleg@...hat.com>, "Vignesh
 Balasubramanian" <vigbalas@....com>
Subject: Re: [PATCH v7 1/6] x86/fpu/xstate: Differentiate default features
 for host and guest FPUs

On Wed, May 21, 2025 at 09:49:48AM -0700, Sean Christopherson wrote:
>On Mon, May 12, 2025, Chao Gao wrote:
>> @@ -772,6 +776,21 @@ static void __init fpu__init_disable_system_xstate(unsigned int legacy_size)
>>  	fpstate_reset(x86_task_fpu(current));
>>  }
>>  
>> +static void __init init_default_features(u64 kernel_max_features, u64 user_max_features)
>> +{
>> +	u64 kfeatures = kernel_max_features;
>> +	u64 ufeatures = user_max_features;
>> +
>> +	/* Default feature sets should not include dynamic xfeatures. */
>> +	kfeatures &= ~XFEATURE_MASK_USER_DYNAMIC;
>> +	ufeatures &= ~XFEATURE_MASK_USER_DYNAMIC;
>> +
>> +	fpu_kernel_cfg.default_features = kfeatures;
>> +	fpu_user_cfg.default_features   = ufeatures;
>> +
>> +	guest_default_cfg.features      = kfeatures;
>> +}
>> +
>>  /*
>>   * Enable and initialize the xsave feature.
>>   * Called once per system bootup.
>> @@ -854,12 +873,8 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
>>  	fpu_user_cfg.max_features = fpu_kernel_cfg.max_features;
>>  	fpu_user_cfg.max_features &= XFEATURE_MASK_USER_SUPPORTED;
>>  
>> -	/* Clean out dynamic features from default */
>> -	fpu_kernel_cfg.default_features = fpu_kernel_cfg.max_features;
>> -	fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;
>> -
>> -	fpu_user_cfg.default_features = fpu_user_cfg.max_features;
>> -	fpu_user_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;
>> +	/* Now, given maximum feature set, determine default values */
>> +	init_default_features(fpu_kernel_cfg.max_features, fpu_user_cfg.max_features);
>
>Passing in max_features is rather odd.  I assume the intent is to capture the
>dependencies, but that falls apart by the end of series as the guest features
>are initialized as:
>
>	guest_default_cfg.features      = kfeatures | xfeatures_mask_guest_supervisor();
>
>where xfeatures_mask_guest_supervisor() sneakily consumes fpu_kernel_cfg.max_features,
>the very field this patch deliberately avoids consuming directly.
>
>  static inline u64 xfeatures_mask_guest_supervisor(void)
>  {
>	return fpu_kernel_cfg.max_features & XFEATURE_MASK_GUEST_SUPERVISOR;
>  }
>

Indeed, it is odd. And your suggestion looks good to me. Thanks.

I will fix this and the comment issue you pointed out and post a new version.