Date: Wed, 3 Jun 2015 19:43:34 +0300
From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: peterhuewe@gmx.de, safford@us.ibm.com,
        Marcel Selhorst <tpmdd@selhorst.net>,
        "moderated list:TPM DEVICE DRIVER" 
	<tpmdd-devel@lists.sourceforge.net>,
        open list <linux-kernel@vger.kernel.org>,
        richard.l.maliszewski@intel.com
Subject: Re: [PATCH] tpm: introduce struct tpm_buf
Message-ID: <20150603164334.GA2327@jsakkine-mobl1>
References: <1433250262-17200-1-git-send-email-jarkko.sakkinen@linux.intel.com>
 <20150602181315.GE17776@obsidianresearch.com>
 <20150603122331.GA3542@jsakkine-mobl1>
 <20150603161128.GB12073@obsidianresearch.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150603161128.GB12073@obsidianresearch.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2175
Lines: 55

On Wed, Jun 03, 2015 at 10:11:28AM -0600, Jason Gunthorpe wrote:
> On Wed, Jun 03, 2015 at 03:23:31PM +0300, Jarkko Sakkinen wrote:
> > On Tue, Jun 02, 2015 at 12:13:15PM -0600, Jason Gunthorpe wrote:
> > > On Tue, Jun 02, 2015 at 04:04:22PM +0300, Jarkko Sakkinen wrote:
> > > > +/* A string buffer type for constructing TPM commands. This is based on the
> > > > + * code in security/keys/trusted.h.
> > > > + */
> > > > +
> > > > +#define TPM_BUF_SIZE 512
> > > > +
> > > > +struct tpm_buf {
> > > > +	u8 data[TPM_BUF_SIZE];
> > > This should be u32 or u64 to guarentee correct alignment for the
> > > casting.
> > 
> > Good catch. The functions where this might cause trouble are *_length()
> > and *_tag().
> > 
> > In other places misalignment should not cause any regressions since data
> > is not directly assigned to the buffer with pointer casting.
> > 
> > I would prefer to fix by changing *_length() and *_tag() to copy the
> > value to a local variable and return that. It's a fail safe way and here
> > the performance is not an issue.
> 
> I would change the type, that is very simple and will improve
> performance of the memcpy as well.
> 
> It actually isn't a problem for the casts to tpm_input_header - any
> structure marked __packed will cause the compiler to assume that the
> entire structure is unaligned and code gen accordingly, so the cast
> will always work, but on x86 it will be more efficient if the array
> is aligned.
> 
> Ideally we could change to something like:
>  __attribute__((packed,aligned(4))) 
> 
> Instead of __packed which will help the compiler minimize unaligned
> load instructions..

I realized basically the same what you said here when I refined the
patch (read this email after sending v2) :) I decided that align to 8
bytes.

I don't think the packed attribute is needed here because now I cast
the beginning to tpm_input_header, which is a packed struct.

> Jason

/Jarkko
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/