linux-kernel - Re: [PATCH v9 05/26] x86/fpu/xstate: Add new variables to indicate dynamic XSTATE buffer size

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Date:   Thu, 12 Aug 2021 17:03:15 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     luto@...nel.org, tglx@...utronix.de, mingo@...nel.org,
        x86@...nel.org, len.brown@...el.com, dave.hansen@...el.com,
        thiago.macieira@...el.com, jing2.liu@...el.com,
        ravi.v.shankar@...el.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v9 05/26] x86/fpu/xstate: Add new variables to indicate
 dynamic XSTATE buffer size

On Fri, Jul 30, 2021 at 07:59:36AM -0700, Chang S. Bae wrote:
> @@ -167,8 +158,10 @@ static void __init fpu__init_task_struct_size(void)
>  	/*
>  	 * Add back the dynamically-calculated register state
>  	 * size.
> +	 *
> +	 * Use the minimum size as embedded to task_struct.

embedded in...

> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 74e608c6ad6c..12caf1a56ce0 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -77,12 +77,51 @@ static unsigned int xstate_comp_offsets[XFEATURE_MAX] __ro_after_init =
>  static unsigned int xstate_supervisor_only_offsets[XFEATURE_MAX] __ro_after_init =
>  	{ [ 0 ... XFEATURE_MAX - 1] = -1};
>  
> -/*
> - * The XSAVE area of kernel can be in standard or compacted format;
> - * it is always in standard format for user mode. This is the user
> - * mode standard format size used for signal and ptrace frames.
> +/**
> + * struct fpu_xstate_buffer_config - xstate buffer configuration
> + * @max_size:			The CPUID-enumerated all-feature "maximum" size
> + *				for xstate per-task buffer.
> + * @min_size:			The size to fit into the statically-allocated
> + *				buffer. With dynamic states, this buffer no longer
> + *				contains all the enabled state components.
> + * @user_size:			The size of user-space buffer for signal and
> + *				ptrace frames, in the non-compacted format.
>   */
> -unsigned int fpu_user_xstate_size __ro_after_init;
> +struct fpu_xstate_buffer_config {
> +	unsigned int min_size, max_size;
> +	unsigned int user_size;
> +};
> +
> +static struct fpu_xstate_buffer_config buffer_config __ro_after_init;

I know I had asked for the accessors below but if this is going to be
read-only after init and is not going to change for duration of the
system lifetime, then you don't really need those accessors.

I.e., you can do

struct fpu_xstate_buffer_config {
	unsigned int min_size, max_size;
	unsigned int user_size;
};

static struct fpu_xstate_buffer_config fpu_buf_cfg __ro_after_init;

and then access those values through fpu_buf_cfg.<value>

Thx.

> +
> +unsigned int get_xstate_config(enum xstate_config cfg)
> +{
> +	switch (cfg) {
> +	case XSTATE_MIN_SIZE:
> +		return buffer_config.min_size;
> +	case XSTATE_MAX_SIZE:
> +		return buffer_config.max_size;
> +	case XSTATE_USER_SIZE:
> +		return buffer_config.user_size;
> +	default:
> +		return 0;
> +	}
> +}
> +EXPORT_SYMBOL_GPL(get_xstate_config);
> +
> +void set_xstate_config(enum xstate_config cfg, unsigned int value)
> +{
> +	switch (cfg) {
> +	case XSTATE_MIN_SIZE:
> +		buffer_config.min_size = value;
> +		break;
> +	case XSTATE_MAX_SIZE:
> +		buffer_config.max_size = value;
> +		break;
> +	case XSTATE_USER_SIZE:
> +		buffer_config.user_size = value;
> +	}
> +}

-- 
Regards/Gruss,
    Boris.

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