linux-kernel - Re: [PATCH v3 06/10] x86/fpu/xstate: Initialize guest perm with fpu_guest

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Message-ID: <Z8u2EWAkqQ+6O5i3@intel.com>
Date: Sat, 8 Mar 2025 11:14:25 +0800
From: Chao Gao <chao.gao@...el.com>
To: Dave Hansen <dave.hansen@...el.com>
CC: <tglx@...utronix.de>, <x86@...nel.org>, <seanjc@...gle.com>,
	<pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<peterz@...radead.org>, <rick.p.edgecombe@...el.com>,
	<weijiang.yang@...el.com>, <john.allen@....com>, <bp@...en8.de>
Subject: Re: [PATCH v3 06/10] x86/fpu/xstate: Initialize guest perm with
 fpu_guest_cfg

On Fri, Mar 07, 2025 at 10:14:19AM -0800, Dave Hansen wrote:
>On 3/7/25 08:41, Chao Gao wrote:
>> From: Yang Weijiang <weijiang.yang@...el.com>
>> 
>> Use the new fpu_guest_cfg to initialize guest permissions.
>
>Background, please.
>
>What are the guest permissions currently set to? Why does it need to change?

Ok. Will add:

Currently, fpu->guest_perm is copied from fpu->perm, which is derived from
fpu_kernel_cfg. To separate the guest FPU from the kernel FPU, switch to use
the new fpu_guest_cfg to initialize guest permissions. This ensures that any
future changes to fpu_guest_cfg will automatically update the guest permissions

The __user_state_size is tied to existing uAPIs, so it remains unchanged.

>
>> Note fpu_guest_cfg and fpu_kernel_cfg remain the same for now. So there
>> is no functional change.
>> 
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> [Gao Chao: Extrace this from the previous patch ]
>> Signed-off-by: Chao Gao <chao.gao@...el.com>
>> ---
>>  arch/x86/kernel/fpu/core.c | 11 +++++++++--
>>  1 file changed, 9 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
>> index b0c1ef40d105..d7ae684adbad 100644
>> --- a/arch/x86/kernel/fpu/core.c
>> +++ b/arch/x86/kernel/fpu/core.c
>> @@ -534,8 +534,15 @@ void fpstate_reset(struct fpu *fpu)
>>  	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;
>>  	fpu->perm.__state_size		= fpu_kernel_cfg.default_size;
>>  	fpu->perm.__user_state_size	= fpu_user_cfg.default_size;
>> -	/* Same defaults for guests */
>> -	fpu->guest_perm = fpu->perm;
>> +
>> +	/* Guest permission settings */
>> +	fpu->guest_perm.__state_perm	= fpu_guest_cfg.default_features;
>> +	fpu->guest_perm.__state_size	= fpu_guest_cfg.default_size;
>> +	/*
>> +	 * Set guest's __user_state_size to fpu_user_cfg.default_size so that
>> +	 * existing uAPIs can still work.
>> +	 */
>
>I suspect that readers here will understand that this line:
>
>> +	fpu->guest_perm.__user_state_size = fpu_user_cfg.default_size;
>>  }
>
>means "Set guest's __user_state_size to fpu_user_cfg.default_size". The
>comment basically just literally restates what the code does. That part
>of the comment doesn't add value.

Will drop this useless comment.