[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options

[Anthony Liguori]

On 03/01/2010 01:27 PM, Michael S. Tsirkin wrote:
> On Sun, Feb 28, 2010 at 10:39:21PM +0000, Paul Brook wrote:
>    
> > > I'm sympathetic to your arguments though.  As qemu is today, the above
> > > is definitely the right thing to do.  But ram is always ram and ram
> > > always has a fixed (albeit non-linear) mapping within a guest.
> > >        
> > I think this assumption is unsafe. There are machines where RAM mappings can
> > change. It's not uncommon for a chip select (i.e. physical memory address
> > region) to be switchable to several different sources, one of which may be
> > RAM.  I'm pretty sure this functionality is present (but not actually
> > implemented) on some of the current qemu targets.
> >
> > I agree that changing RAM mappings under an active DMA is a fairly suspect
> > thing to do. However I think we need to avoid cache mappings between separate
> > DMA transactions i.e. when the guest can know that no DMA will occur, and
> > safely remap things.
> >
> > I'm also of the opinion that virtio devices should behave the same as any
> > other device. i.e. if you put a virtio-net-pci device on a PCI bus behind an
> > IOMMU, then it should see the same address space as any other PCI device in
> > that location.
> >      
> It already doesn't. virtio passes physical memory addresses
> to device instead of DMA addresses.
>    

That's technically a bug.

> > Apart from anything else, failure to do this breaks nested
> > virtualization.
> >      
> Assigning PV device in nested virtualization? It could work, but not
> sure what the point would be.
>    

It misses the point really.

vhost-net is not a device model and it shouldn't have to care about 
things like PCI IOMMU.  If we did ever implement a PCI IOMMU, then we 
would perform ring translation (or not use vhost-net).

Regards,

Anthony Liguori

> >   While qemu doesn't currently implement an IOMMU, the DMA
> > interfaces have been designed to allow it.
> >
> >      
> > > void cpu_ram_add(target_phys_addr_t start, ram_addr_t size);
> > >        
> > We need to support aliased memory regions. For example the ARM RealView boards
> > expose the first 256M RAM at both address 0x0 and 0x70000000. It's also common
> > for systems to create aliases by ignoring certain address bits. e.g. each sim
> > slot is allocated a fixed 256M region. Populating that slot with a 128M stick
> > will cause the contents to be aliased in both the top and bottom halves of
> > that region.
> >
> > Paul
> >