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

[Michael S. Tsirkin]

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.

> >>  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.
> 
> The general case is dma'ing a structure, not a single field.  That
> doesn't mean we shouldn't have a helper.
> 
> -- 
> error compiling committee.c: too many arguments to function