linux-kernel - Re: PATCH: Update disable_IO_APIC to use 8-bit destination field (X86

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Message-Id: <1169132294.6665.2.camel@ustr-romerbm-2.na.uis.unisys.com>
Date:	Thu, 18 Jan 2007 09:58:14 -0500
From:	Benjamin Romer <benjamin.romer@...sys.com>
To:	vgoyal@...ibm.com
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org
Subject: Re: PATCH: Update disable_IO_APIC to use 8-bit destination field 
	(X86_64)

On Thu, 2007-01-18 at 13:30 +0530, Vivek Goyal wrote:
> On Thu, Jan 18, 2007 at 12:10:55AM -0700, Eric W. Biederman wrote:
> > Vivek Goyal <vgoyal@...ibm.com> writes:
> > >
> > > Or how about making physical_dest field also 8bit like logical_dest field.
> > > This will work both for 4bit and 8bit physical apic ids at the same time
> > > code becomes more intutive and it is easier to know whether IOAPIC is being
> > > put in physical or destination logical mode.
> > 
> > Exactly what I was trying to suggest.
> > 
> > Looking closer at the code I think it makes sense to just kill the union and
> > stop the discrimination between physical and logical modes and just have a
> > dest field in the structure.  Roughly as you were suggesting at first.
> > 
> > The reason we aren't bitten by this on a regular basis is the normal code
> > path uses logical.logical_dest in both logical and physical modes.
> > Which is a little confusing.
> > 
> > Since there really isn't a distinction to be made we should just stop
> > trying, which will make maintenance easier :)
> > 
> > Currently there are several non-common case users of physical_dest
> > that are probably bitten by this problem under the right
> > circumstances.
> > 
> > So I think we should just make the structure:
> > 
> > struct IO_APIC_route_entry {
> > 	__u32	vector		:  8,
> > 		delivery_mode	:  3,	/* 000: FIXED
> > 					 * 001: lowest prio
> > 					 * 111: ExtINT
> > 					 */
> > 		dest_mode	:  1,	/* 0: physical, 1: logical */
> > 		delivery_status	:  1,
> > 		polarity	:  1,
> > 		irr		:  1,
> > 		trigger		:  1,	/* 0: edge, 1: level */
> > 		mask		:  1,	/* 0: enabled, 1: disabled */
> > 		__reserved_2	: 15;
> > 
> > 	__u32	__reserved_3	: 24,
> > 		__dest		:  8;
> > } __attribute__ ((packed));
> > 
> > And fixup the users.  This should keep us from getting bit by this bug
> > in the future.  Like when people start introducing support for more
> > than 256 cores and the low 24bits start getting used.
> > 
> > Or when someone new starts working on the code and thinks the fact
> > the field name says logical we are actually using the apic in logical
> > mode.
> 
> This makes perfect sense to me. Ben, interested in providing a patch 
> for this?
> 
> Thanks
> Vivek

Sure! I'll fix it up and get a revised patch out as soon as I can. :) 

-- Ben

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