linux-kernel - Re: [PATCH v7 3/6] x86/fpu: Initialize guest fpstate and FPU pseudo container from guest defaults

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Message-ID: <aCISETY5C7V6Pfyt@intel.com>
Date: Mon, 12 May 2025 23:21:53 +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>, Eric Biggers <ebiggers@...gle.com>, Kees Cook
	<kees@...nel.org>
Subject: Re: [PATCH v7 3/6] x86/fpu: Initialize guest fpstate and FPU pseudo
 container from guest defaults

On Mon, May 12, 2025 at 07:13:08AM -0700, Sean Christopherson wrote:
>On Mon, May 12, 2025, Chao Gao wrote:
>> @@ -535,10 +538,20 @@ void fpstate_init_user(struct fpstate *fpstate)
>>  
>>  static void __fpstate_reset(struct fpstate *fpstate, u64 xfd)
>>  {
>> -	/* Initialize sizes and feature masks */
>> -	fpstate->size		= fpu_kernel_cfg.default_size;
>> +	/*
>> +	 * Initialize sizes and feature masks. Supervisor features and
>> +	 * sizes may diverge between guest FPUs and host FPUs, whereas
>> +	 * user features and sizes are always identical the same.
>
>Pick of of "identical" or "the same" :-)

Sure.

>
>And maybe explain why supervisor features can diverge, while the kernel ensures
>user features are identical?  Ditto for the XFD divergence.  E.g. I think this
>would be accurate (though I may be reading too much into user features):
>
>	/*
>	 * Supervisor features (and thus sizes) may diverge between guest FPUs
>	 * and host FPUs, as some supervisor features are supported for guests
>	 * despite not being utilized by the host.  User features and sizes are
>	 * always identical, which allows for common guest and userspace ABI.
>	 *
>	 * For the host, set XFD to the kernel's desired initialization value.
>	 * For guests, set XFD to its architectural RESET value.
>	 */

Yea, this looks much better.