linux-kernel - Re: [RFC PATCH 22/22] x86/fpu/xstate: Introduce boot-parameters for control some state component support

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Date:   Thu, 1 Oct 2020 19:09:48 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>, tglx@...utronix.de,
        mingo@...nel.org, bp@...e.de, luto@...nel.org, x86@...nel.org
Cc:     len.brown@...el.com, dave.hansen@...el.com, jing2.liu@...el.com,
        ravi.v.shankar@...el.com, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH 22/22] x86/fpu/xstate: Introduce boot-parameters for
 control some state component support

Hi--

On 10/1/20 1:39 PM, Chang S. Bae wrote:
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a1068742a6df..742167c6f789 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5838,6 +5838,21 @@
>  			which allow the hypervisor to 'idle' the guest on lock
>  			contention.
>  
> +	xstate.enable=	[X86-64]
> +	xstate.disable=	[X86-64]
> +			The kernel is compiled with a default xstate bitmask --
> +			enabling it to use the XSAVE hardware to efficiently
> +			save and restore thread states on context switch.
> +			xstate.enable allows adding to that default mask at
> +			boot-time without recompiling the kernel just to support
> +			the new thread state. (Note that the kernel will ignore
> +			any bits in the mask that do not correspond to features
> +			that are actually available in CPUID)  xstate.disable

			                             in CPUID.)

> +			allows clearing bits in the default mask, forcing the
> +			kernel to forget that it supports the specified thread
> +			state. When a bit set for both, the kernel takes
> +			xstate.disable in a priority.

			               as a priority.


What do these bitmasks look like?  what do the bits mean?
Where does a user find this info?


thanks.
-- 
~Randy