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

[Michael S. Tsirkin]

On Tue, Oct 18, 2011 at 12:46:50PM +1100, David Gibson wrote:
> On Sun, Oct 16, 2011 at 03:15:53PM +0200, Avi Kivity wrote:
> > On 10/14/2011 04:14 AM, David Gibson wrote:
> > > > Virtio is a very, very special case.  virtio requires coherent RAM 
> > > > access.
> > >
> > > Right.  Virtio's access to memory is *not* emulated PCI DMA, it's
> > > god-like hypervisor access to guest system memory.  It should
> > > correctly bypass any IOMMU, and so should remain as
> > > cpu_physical_memory_rw() or the atomic accessors, rather than being
> > > converted to this new API.
> > 
> > virtio should definitely not bypass an iommu.
> 
> So, I just had a chat with Rusty about this.  Perhaps it shouldn't,
> but it does.  The spec is in terms of guest physical addresses, not
> bus/DMA addresses, and more to the point the Linux driver does *not*
> do the necessary dma_map() and unmap operations to treat this as a PCI
> DMA.  So like it or not, god-like hypervisor access rather than
> emulated PCI DMA is what it does.

Fine, but I'm convinced virtio is not unique in that
it wants atomic accesses.

I just looked at hw/rtl8139.c as one example.
It uses a high bit in a 32 bit register to signal
descriptor ownership. Thus we need to read that
bit first, or read the register atomically.

Current code does cpu_physical_memory_read
which does neither of these things, but
it seems to be a bug. An atomic load would
be the best solution.

-- 
MST