Re: [Qemu-devel] [PATCH] ahci: map memory via device's address space instead of address_space_memory

[Michael S. Tsirkin]

On Thu, Jul 03, 2014 at 11:41:29AM +0200, Paolo Bonzini wrote:
> Il 03/07/2014 10:43, Jan Kiszka ha scritto:
> >On 2014-07-03 10:26, Le Tan wrote:
> >>In map_page() in hw/ide/ahci.c, replace cpu_physical_memory_map() and
> >>cpu_physical_memory_unmap() with dma_memory_map() and dma_memory_unmap(),
> >>because ahci devices should not access memory directly but via their address
> >>space. Add an AddressSpace parameter to map_page(). In order to call
> >>map_page(), we should pass the AHCIState.as as the AddressSpace argument.
> >
> >BTW, when doing "git grep cpu_physical_memory_map hw", there are some
> >more cases that should be checked (for x86). I suppose vhost is
> >incompatible with an IOMMU, but plain virtio should work, same for vmxnet.
> 
> I think PPC folks explicitly wanted virtio to bypass the IOMMU, probably in
> order to get vhost running.  It seems like a bad idea to me, but who am I...
> 
> Paolo

The argument went like this: IOMMU slows things down. People
might want to run a VM where nested virt is *possible*
so IOMMU has to be enabled but still get fast IO for the rest of
the guest.

-- 
MST