linux-kernel - Re: [Xen-devel] [PATCH 1/9] x86/boot: enumerate documentation for the x86 hardware

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Message-ID: <56C71B51.20109@citrix.com>
Date:	Fri, 19 Feb 2016 13:40:33 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	"Luis R. Rodriguez" <mcgrof@...nel.org>, <bp@...en8.de>
CC:	<rusty@...tcorp.com.au>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, <luto@...capital.net>,
	<xen-devel@...ts.xensource.com>, <david.vrabel@...rix.com>,
	<boris.ostrovsky@...cle.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [Xen-devel] [PATCH 1/9] x86/boot: enumerate documentation for the
 x86 hardware_subarch

On 19/02/16 13:08, Luis R. Rodriguez wrote:
> Although hardware_subarch has been in place since the x86 boot
> protocol 2.07 it hasn't been used much. Enumerate current possible
> values to avoid misuses and help with semantics later at boot
> time should this be used further.
> 
> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>
> ---
>  arch/x86/include/uapi/asm/bootparam.h | 32 +++++++++++++++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
> index 329254373479..dbfb9406436b 100644
> --- a/arch/x86/include/uapi/asm/bootparam.h
> +++ b/arch/x86/include/uapi/asm/bootparam.h
> @@ -157,7 +157,37 @@ struct boot_params {
>  	__u8  _pad9[276];				/* 0xeec */
>  } __attribute__((packed));
>  
> -enum {
> +/**
> + * enum x86_hardware_subarch - x86 hardware subarchitecture
> + *
> + * The x86 hardware_subarch and hardware_subarch_data were added as of the x86
> + * boot protocol 2.07 to help distinguish and supports custom x86 boot
> + * sequences. This enum represents accepted values for the x86
> + * hardware_subarch. Custom x86 boot sequences (not X86_SUBARCH_PC) do not have
> + * or simply do not make use of natural stubs like BIOS or EFI, the
> + * hardware_subarch can be used on the Linux entry path to revector to a
> + * subarchitecture stub when needed. This subarchitecture stub can be used to
> + * set up Linux boot parameters or for special care to account for nonstandard
> + * handling of page tables.

This documentation reads like a plan for future implementation.  Is this
the level of documentation that is needed here?

Also, "revector to a subarchitecture stub" is a rather odd way of saying
"call a subarch-specific stub".

David