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

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Date:	Fri, 19 Feb 2016 15:40:19 +0100
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...nel.org>, bp@...en8.de,
	rusty@...tcorp.com.au, x86@...nel.org,
	linux-kernel@...r.kernel.org, luto@...capital.net,
	xen-devel@...ts.xensource.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 Fri, Feb 19, 2016 at 01:40:33PM +0000, David Vrabel wrote:
> 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".

I took feedback directly from hpa's replies about what the subarch is from
my original linker table series. So its not me concocting this. It also
shows how your own lack of clarify on this isn't doing us any good either,
and its precisely why I am documenting this now.

The lack of semantics here has gotten us intro trouble as it is not allowing
us to tool in proactive solutions to prevent bugs. My goal is first to
clarify some definitions, and then use them and tool them to proactiveley
avoid bugs.

  Luis