linux-kernel - Re: [patch 5/6] ps3: BD/DVD/CD-ROM Storage Driver

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Date:	Fri, 13 Jul 2007 16:51:26 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
Cc:	James Bottomley <James.Bottomley@...elEye.com>,
	Paul Mackerras <paulus@...ba.org>,
	Alessandro Rubini <rubini@...ion.unipv.it>,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org,
	Geoff Levand <geoffrey.levand@...sony.com>
Subject: Re: [patch 5/6] ps3: BD/DVD/CD-ROM Storage Driver

On Fri, Jul 13 2007, Geert Uytterhoeven wrote:
> On Fri, 13 Jul 2007, Jens Axboe wrote:
> > On Fri, Jul 13 2007, James Bottomley wrote:
> > > On Wed, 2007-07-04 at 15:22 +0200, Geert Uytterhoeven wrote:
> > > > +                       kaddr = kmap_atomic(sgpnt->page, KM_USER0);
> > > > +                       if (!kaddr)
> > > > +                               return -1;
> > > > +                       len = sgpnt->length;
> > > > +                       if ((req_len + len) > buflen) {
> > > > +                               active = 0;
> > > > +                               len = buflen - req_len;
> > > > +                       }
> > > > +                       memcpy(kaddr + sgpnt->offset, buf + req_len,
> > > > len);
> > > > +                       kunmap_atomic(kaddr, KM_USER0);
> > > 
> > > This isn't a SCSI objection, but this sequence appears several times in
> > > this driver.  It's wrong for a non-PIPT architecture (and I believe the
> > > PS3 is VIPT) because you copy into the kernel alias for the page, which
> > > dirties the line in the cache of that alias (the user alias cache line
> > > was already invalidated).  However, unless you flush the kernel alias to
> > > main memory, the user could read stale data.  The way this is supposed
> > > to be done is to do a 
> > > 
> > > flush_kernel_dcache_page(kaddr)
> > > 
> > > before doing the kunmap.
> > > 
> > > Otherwise it looks OK from the SCSI point of view.
> 
> kmap() just returns page_address() on ppc64, as there's no highmem.
> kunmap() is a no-op.
> 
> So technically I could just use page_address() directly, but Christoph wanted
> me to keep the kmap()/kunmap() sequence because it's considered a good
> practice.

If you have the kmap sequence there, put the flush in as well. People
copy code, you know... Or put a big comment explaining why it isn't
needed.

> > Well, even worse is that fact that it's using KM_USER0 from interrupt
> > context.
> 
> So should I replace it by e.g. KM_IRQ0?
> I'm not so familiar with these parts, and I couldn't find what these values
> really mean.

You corrupt data, using KM_USER0 from interrupt context. So it's a big
flaw right now. Use KM_IRQ0 for code where interrupts are always
disabled.

-- 
Jens Axboe

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