Re: [PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to control state component support

[Randy Dunlap]

On 2/21/21 10:56 AM, Chang S. Bae wrote:
> "xstate.disable=0x60000" will disable AMX on a system that has AMX compiled
> into XFEATURE_MASK_USER_ENABLED.
> 
> "xstate.enable=0x60000" will enable AMX on a system that does NOT have AMX
> compiled into XFEATURE_MASK_USER_ENABLED (assuming the kernel is new enough
> to support this feature).
> 
> Rename XFEATURE_MASK_USER_SUPPORTED to XFEATURE_MASK_USER_ENABLED to be
> aligned with the new parameters.
> 
> While this cmdline is currently enabled only for AMX, it is intended to be
> easily enabled to be useful for future XSAVE-enabled features.
> 

Hi,
Can we tell people (in this Doc file) where to look up the values that can be
used in xstate.enable and xstate.disable?

thanks.

> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> Changes from v3:
> * Fixed a few typos. (Randy Dunlap)
> 
> Changes from v2:
> * Changed the kernel tainted when any unknown state is enabled. (Andy
>   Lutomirski)
> * Simplified the cmdline handling.
> * Edited the changelog.
> 
> Changes from v1:
> * Renamed the user state mask define (Andy Lutomirski and Dave Hansen)
> * Changed the error message (Dave Hansen)
> * Fixed xfeatures_mask_user()
> * Rebased the upstream kernel (5.10) -- revived the param parse function
> ---
>  .../admin-guide/kernel-parameters.txt         | 15 +++++
>  arch/x86/include/asm/fpu/types.h              |  6 ++
>  arch/x86/include/asm/fpu/xstate.h             | 24 +++----
>  arch/x86/kernel/fpu/init.c                    | 65 +++++++++++++++++--
>  4 files changed, 93 insertions(+), 17 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a10b545c2070..ec79f63979a4 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6014,6 +6014,21 @@
>  			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
> +			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 as a priority.
> +
>  	xirc2ps_cs=	[NET,PCMCIA]
>  			Format:
>  			<irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]



-- 
~Randy