Re: [PATCH qemu-kvm] device assignment: default requires IOMMU

[Simon Horman <horms@verge.net.au>]

On Thu, Dec 24, 2009 at 02:56:00PM +0800, Sheng Yang wrote:
> On Thursday 24 December 2009 14:51:23 Simon Horman wrote:
> > On Thu, Dec 24, 2009 at 01:45:34AM +0100, Alexander Graf wrote:
> > > Am 23.12.2009 um 23:40 schrieb Chris Wright <chrisw@sous-sol.org>:
> > > >[ resend, fixing email header, sorry for duplicate ]
> > > >
> > > >The default mode for device assignment is to rely on an IOMMU for
> > > >proper translations and a functioning device in the guest.  The
> > > >current
> > > >logic makes this requirement advisory, and simply disables the request
> > > >for IOMMU if one is not found on the host.  This makes for a confused
> > > >user when the device assignment appears to work, but the device in the
> > > >guest is not functioning  (I've seen about a half-dozen reports with
> > > >this failure mode).
> > > >
> > > >Change the logic such that the default requires the IOMMU.  Period.
> > > >If the host does not have an IOMMU, device assignment will fail.
> > > >
> > > >This is a user visible change, however I think the current
> > > >situation is
> > > >simply broken.
> > > >
> > > >And, of course, disabling the IOMMU requirement using the old:
> > > >
> > > >  -pcidevice host=[addr],dma=none
> > > >
> > > >or the newer:
> > > >
> > > >  -device pci-assign,host=[addr],iommu=0
> > > >
> > > >will do what it always did (not require an IOMMU, and fail to work
> > > >properly).
> > >
> > > Yay!
> > 
> > Sounds good to me. Though I am curious to know the reasoning
> > behind the current logic.
> > 
> Sounds pretty good. :)
> 
> I think maybe it due to we are interested in implementing PV DMA?

Ok, that would explain it.