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.
> I think endian conversion is best made explicit in virtio (like
> e1000 does explicit conversions to little endian).
That's certainly possible. Though it's hard to see why duplicating e.g.
static void e100_stw_le_phys(target_phys_addr_t addr, uint16_t val)
{
val = cpu_to_le16(val);
cpu_physical_memory_write(addr,&val, sizeof(val));
}
is a better idea than a central utility that does this.
Maybe the address is not guaranteed to be aligned in the e100
case.