Re: [Qemu-devel] [PATCH 05/14] eepro100: Use PCI DMA stub functions

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 03, 2011 at 04:16:34PM +1100, David Gibson wrote:
> On Wed, Nov 02, 2011 at 09:16:34AM +0200, Michael S. Tsirkin wrote:
> > On Mon, Oct 31, 2011 at 05:06:49PM +1100, David Gibson wrote:
> > > From: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
> [snip]
> > > @@ -744,21 +713,26 @@ static void dump_statistics(EEPRO100State * s)
> > >       * values which really matter.
> > >       * Number of data should check configuration!!!
> > >       */
> > > -    cpu_physical_memory_write(s->statsaddr, &s->statistics, s->stats_size);
> > > -    e100_stl_le_phys(s->statsaddr + 0, s->statistics.tx_good_frames);
> > > -    e100_stl_le_phys(s->statsaddr + 36, s->statistics.rx_good_frames);
> > > -    e100_stl_le_phys(s->statsaddr + 48, s->statistics.rx_resource_errors);
> > > -    e100_stl_le_phys(s->statsaddr + 60, s->statistics.rx_short_frame_errors);
> > > +    pci_dma_write(&s->dev, s->statsaddr,
> > > +                  (uint8_t *) &s->statistics, s->stats_size);
> > > +    stl_le_pci_dma(&s->dev, s->statsaddr + 0,
> > > +                   s->statistics.tx_good_frames);
> > > +    stl_le_pci_dma(&s->dev, s->statsaddr + 36,
> > > +                   s->statistics.rx_good_frames);
> > > +    stl_le_pci_dma(&s->dev, s->statsaddr + 48,
> > > +                   s->statistics.rx_resource_errors);
> > > +    stl_le_pci_dma(&s->dev, s->statsaddr + 60,
> > > +                   s->statistics.rx_short_frame_errors);
> > 
> > This might introduce a bug: stlXX APIs assume aligned addresses,
> > an address in statsaddr is user-controlled so I'm not sure
> > it's always aligned.
> > 
> > Why isn't the patch simply replacing cpu_physical_memory_read
> > with pci_XXX ? Any cleanups should be done separately.
> 
> Because it seemed like a good idea at the time.  When I first wrote
> this, the possibility of unaligned addresses wasn't obvious to me.
> So, I'm working on fixing this now.  I can take one of two approaches:
> 
>  - Simply revert this part of the change, reinstate the e100_stl
> functions as calling into dma_write().
> 
>  - Alter the stX_dma() functions to work for unaligned addresses (by
> falling back to dma_rw() in that case).  This is a little more
> involved but might make device writing safer in future.

Yes but then we lose the atomicity guarantee. So this might
still result in subtle emulation bugs.

> Anthony, Michael, any preferred direction here?

For 1.0 I'd go for option 1 as the simplest.

> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson