Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA

[David Gibson]

On Sun, Oct 02, 2011 at 02:14:28PM +0200, Michael S. Tsirkin wrote:
> On Sun, Oct 02, 2011 at 02:01:10PM +0200, Avi Kivity wrote:
> > On 10/02/2011 01:17 PM, Michael S. Tsirkin wrote:
> > >On Sun, Oct 02, 2011 at 12:58:35PM +0200, Avi Kivity wrote:
> > >>  On 10/02/2011 12:52 PM, Michael S. Tsirkin wrote:
> > >>  >On Sun, Oct 02, 2011 at 12:29:08PM +0200, Avi Kivity wrote:
> > >>  >>   On 10/02/2011 12:25 PM, Michael S. Tsirkin wrote:
> > >>  >>   >On Mon, Sep 05, 2011 at 02:34:56PM +1000, David Gibson wrote:
> > >>  >>   >>    This patch adds functions to pci.[ch] to perform PCI DMA 
> > >> operations.  At
> > >>  >>   >>    present, these are just stubs which perform directly cpu 
> > >> physical memory
> > >>  >>   >>    accesses.
> > >>  >>   >>
> > >>  >>   >>    Using these stubs, however, distinguishes PCI device DMA 
> > >> transactions from
> > >>  >>   >>    other accesses to physical memory, which will allow PCI 
> > >> IOMMU support to
> > >>  >>   >>    be added in one place, rather than updating every PCI driver 
> > >> at that time.
> > >>  >>   >>
> > >>  >>   >>    That is, it allows us to update individual PCI drivers to 
> > >> support an IOMMU
> > >>  >>   >>    without having yet determined the details of how the IOMMU 
> > >> emulation will
> > >>  >>   >>    operate.  This will let us remove the most bitrot-sensitive 
> > >> part of an
> > >>  >>   >>    IOMMU patch in advance.
> > >>  >>   >>
> > >>  >>   >>    Signed-off-by: David Gibson<address@hidden>
> > >>  >>   >
> > >>  >>   >So something I just thought about:
> > >>  >>   >
> > >>  >>   >all wrappers now go through cpu_physical_memory_rw.
> > >>  >>   >This is a problem as e.g. virtio assumes that
> > >>  >>   >accesses such as stw are atomic. cpu_physical_memory_rw
> > >>  >>   >is a memcpy which makes no such guarantees.
> > >>  >>   >
> > >>  >>
> > >>  >>   Let's change cpu_physical_memory_rw() to provide that guarantee for
> > >>  >>   aligned two and four byte accesses.  Having separate paths just for
> > >>  >>   that is not maintainable.
> > >>  >
> > >>  >Well, we also have stX_phys convert to target native endian-ness
> > >>  >(nop for KVM but not necessarily for qemu).
> > >>  >
> > >>  >So if we do what you suggest, this patch will become more correct, but
> > >>  >it would still need to duplicate the endian-ness work.
> > >>  >
> > >>  >For that reason, I think calling stX_phys and friends from pci
> > >>  >makes more sense - we get more simple inline wrappers
> > >>  >but that code duplication worries me much less than tricky
> > >>  >endian-ness hidden within a macro.
> > >>  >
> > >>
> > >>  Good point.  Though this is really a virtio specific issue since
> > >>  other devices have explicit endianness (not guest dependent).
> > >
> > >Hmm, not entirely virtio specific, some devices use stX macros to do the
> > >conversion.  E.g. stw_be_phys and stl_le_phys are used in several
> > >places.
> > 
> > These are fine - explicit endianness.
> 
> Right. So changing these to e.g. stl_dma and assuming
> LE is default seems like a step backwards.

Um.. why?  PCI is defined by the spec to be LE, so I don't see that we
need explicit endianness versions for PCI helpers.

-- 
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