linux-kernel - Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

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Message-ID: <f53bea9b13bd8351dc9bba5e443d5e4f4934555d.camel@intel.com>
Date: Thu, 24 Apr 2025 22:52:59 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Gao, Chao" <chao.gao@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "ebiggers@...gle.com" <ebiggers@...gle.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "Spassov, Stanislav" <stanspas@...zon.de>,
	"levymitchell0@...il.com" <levymitchell0@...il.com>,
	"samuel.holland@...ive.com" <samuel.holland@...ive.com>, "Li, Xin3"
	<xin3.li@...el.com>, "Yang, Weijiang" <weijiang.yang@...el.com>,
	"mingo@...hat.com" <mingo@...hat.com>, "mlevitsk@...hat.com"
	<mlevitsk@...hat.com>, "john.allen@....com" <john.allen@....com>, "Bae, Chang
 Seok" <chang.seok.bae@...el.com>, "vigbalas@....com" <vigbalas@....com>,
	"hpa@...or.com" <hpa@...or.com>, "peterz@...radead.org"
	<peterz@...radead.org>, "aruna.ramakrishna@...cle.com"
	<aruna.ramakrishna@...cle.com>, "bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for
 host and guest FPUs

On Thu, 2025-04-10 at 15:24 +0800, Chao Gao wrote:
> +
> +	/*
> +	 * @user_size:
> +	 *
> +	 * The default UABI size of the register state buffer in guest
> +	 * FPUs. Includes all supported user features except independent
> +	 * managed features and features which have to be requested by
> +	 * user space before usage.
> +	 */
> +	unsigned int user_size;
> +
> +	/*
> +	 * @features:
> +	 *
> +	 * The default supported features bitmap in guest FPUs. Does not
> +	 * include independent managed features and features which have to
> +	 * be requested by user space before usage.
> +	 */
> +	u64 features;
> +
> +	/*
> +	 * @user_features:
> +	 *
> +	 * Same as @features except only user xfeatures are included.
> +	 */
> +	u64 user_features;
> +};

Tracing through the code, it seems that fpu_user_cfg.default_features and
guest_default_cfg.user_features are the same, leading to
fpu_user_cfg.default_size and guest_default_cfg.user_size being also the same.

In the later patches, it doesn't seem to change the "user" parts. These
configurations end up controlling the default size and features that gets copied
to userspace in KVM_SET_XSAVE. I guess today there is only one default size and
feature set for xstate copied to userspace. The suggestion from Chang was that
it makes the code more readable, but it seems like it also breaks apart a
unified concept for no functional benefit.

Maybe we don't need user_features or user_size here in vcpu_fpu_config? Or did I
get lost somewhere along the way in all the twists and turns that features and
sizes go through.