linux-kernel - Re: [PATCH] printk: Export struct log size and member offsets through vmcoreinfo

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Message-ID: <20120719140826.GD2761@redhat.com>
Date:	Thu, 19 Jul 2012 10:08:26 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Kay Sievers <kay@...y.org>
Cc:	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartmann <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] printk: Export struct log size and member offsets
 through vmcoreinfo

On Thu, Jul 19, 2012 at 09:57:36AM -0400, Vivek Goyal wrote:
> On Thu, Jul 19, 2012 at 11:38:57AM +0200, Kay Sievers wrote:
> > On Wed, Jul 18, 2012 at 7:56 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> > > On Wed, Jul 18, 2012 at 07:27:08PM +0200, Kay Sievers wrote:
> > >> On Wed, Jul 18, 2012 at 7:18 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> > >>
> > >> > Currently I am not exporting log "level" info as that is a bitfield and
> > >> > offsetof() bitfields can't be calculated.
> > >>
> > >> We could make the level the lower 3 bits of the byte, export the byte,
> > >> and define that only 3 bits of the byte are valid? Would that help?
> > >
> > > Yes, that should work. Here is the prototype patch which stores 5 bits
> > > of flag and 3 bits of level in a byte. I have not tested it yet, but
> > > if you like the approach, I will test it.
> > 
> > > -       u8 flags:5;             /* internal record flags */
> > > -       u8 level:3;             /* syslog level */
> > > +       u8 flags_level;         /* 5 bit internal record flags, 3 bits syslog
> > 
> > Looks ok.
> > 
> > If we would swap the 5 + 3 bit field byte declaration, and add
> > __packed, we can still not rely on the level to be consistently the
> > lower 3 bits of the byte, right?
> 

I think I missed your point in last response. Are you saying that retain
bit fields  for flags and level, and add __packed() and that will make sure
level bits are always lowest 3bits? I am really not sure how that is going
to work. Also if you want to add more fields to struct log down the line,
it will be a problem to determine the offset of byte where level bits are
stored. 

Thanks
Vivek
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