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

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Message-ID: <63044dfe-7f44-4a3d-9a27-82c036232241@intel.com>
Date: Fri, 7 Mar 2025 10:14:19 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Chao Gao <chao.gao@...el.com>, tglx@...utronix.de, x86@...nel.org,
 seanjc@...gle.com, pbonzini@...hat.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: 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 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?

> 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.