Re: [PATCH v2 1/1] Documentation/x86: Add the AMX enabling example

[Bagas Sanjaya]

On 6/30/22 05:42, Chang S. Bae wrote:
> Explain steps to enable the dynamic feature with a code example.
> 
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Reviewed-by: Thiago Macieira <thiago.macieira@xxxxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> Changes from v1:
> * Update the description without mentioning CPUID & XGETBV (Dave Hansen).
> ---
>  Documentation/x86/xstate.rst | 42 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst
> index 5cec7fb558d6..c439901419fb 100644
> --- a/Documentation/x86/xstate.rst
> +++ b/Documentation/x86/xstate.rst
> @@ -64,6 +64,48 @@ the handler allocates a larger xstate buffer for the task so the large
>  state can be context switched. In the unlikely cases that the allocation
>  fails, the kernel sends SIGSEGV.
>  
> +AMX TILE_DATA enabling example
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The following steps dynamically enable TILE_DATA:
> +

This should be "Below is the example of how userspace applications enable
TILE_DATA dynamically:"

> +  1. An application first needs to determine the feature support::
> +

Better say "The application first needs to query the kernel for AMX
support".

> +        #include <asm/prctl.h>
> +        #include <sys/syscall.h>
> +        #include <stdio.h>
> +        #include <unistd.h>
> +
> +        #define ARCH_GET_XCOMP_SUPP  0x1021
> +
> +        #define XFEATURE_XTILECFG    17
> +        #define XFEATURE_XTILEDATA   18
> +        #define XFEATURE_MASK_XTILE ((1 << XFEATURE_XTILECFG) | (1 << XFEATURE_XTILEDATA))
> +
> +        unsigned long features;
> +        long rc;
> +
> +        ...
> +
> +        rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
> +
> +        if (!rc && (features & XFEATURE_MASK_XTILE) == XFEATURE_MASK_XTILE)
> +            printf("AMX is available.\n");
> +
> +  2. After determining support for AMX, an application must explicitly ask
> +     permission to use it::
> +

Shorter is "After that,..."

> +        #define ARCH_REQ_XCOMP_PERM  0x1023
> +
> +        ...
> +
> +        rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA);
> +
> +        if (!rc)
> +            printf("AMX is ready for use.\n");
> +
> +Note this example does not include the sigaltstack preparation.
> +

I guess "application" here means userspace application, right?

-- 
An old man doll... just what I always wanted! - Clara