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From: Vivek Goyal <vgoyal@in.ibm.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Benjamin Romer <benjamin.romer@unisys.com>, linux-kernel@vger.kernel.org
Subject: Re: PATCH: Update disable_IO_APIC to use 8-bit destination field (X86_64)
Date: Thu, 18 Jan 2007 13:30:03 +0530	[thread overview]
Message-ID: <20070118080003.GC31860@in.ibm.com> (raw)
In-Reply-To: <m1tzyo7qtc.fsf@ebiederm.dsl.xmission.com>

On Thu, Jan 18, 2007 at 12:10:55AM -0700, Eric W. Biederman wrote:
> Vivek Goyal <vgoyal@in.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

  reply	other threads:[~2007-01-18  8:00 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-01-17 16:46 Benjamin Romer
2007-01-17 19:08 ` Eric W. Biederman
2007-01-18  3:41   ` Vivek Goyal
2007-01-18  4:36     ` Vivek Goyal
2007-01-18  7:10       ` Eric W. Biederman
     [not found]         ` <2 0070118080003.GC31860@in.ibm.com>
2007-01-18  8:00         ` Vivek Goyal [this message]
2007-01-18 14:58           ` Benjamin Romer
2007-01-18 17:23           ` Benjamin Romer
2007-01-18 18:14             ` Eric W. Biederman
     [not found]               ` <1169147619.6665. 11.camel@ustr-romerbm-2.na.uis.unisys.com>
2007-01-18 19:13               ` Benjamin Romer
2007-01-19  3:49                 ` Vivek Goyal
2007-01-19 15:11                   ` Benjamin Romer
2007-01-19 15:43                     ` Randy Dunlap
2007-01-19 17:14                       ` Benjamin Romer
2007-01-19 15:55                     ` Eric W. Biederman

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