Re: Plan for /dev/ioasid RFC v2

Alex Williamson
2021-06-18T15:52:09Z

On Fri, 18 Jun 2021 08:37:35 -0700
"Raj, Ashok" <ashok.raj@intel.com> wrote:

> On Fri, Jun 18, 2021 at 12:15:06PM -0300, Jason Gunthorpe wrote:
> > On Fri, Jun 18, 2021 at 03:47:51PM +0200, Joerg Roedel wrote:
> > > Hi Kevin,
> > >
> > > On Thu, Jun 17, 2021 at 07:31:03AM +0000, Tian, Kevin wrote:
> > > > Now let's talk about the new IOMMU behavior:
> > > >
> > > > - A device is blocked from doing DMA to any resource outside of
> > > > its group when it's probed by the IOMMU driver. This could be a
> > > > special state w/o attaching to any domain, or a new special domain
> > > > type which differentiates it from existing domain types (identity,
> > > > dma, or unmanged). Actually existing code already includes a
> > > > IOMMU_DOMAIN_BLOCKED type but nobody uses it.
> > >
> > > There is a reason for the default domain to exist: Devices which require
> > > RMRR mappings to be present. You can't just block all DMA from devices
> > > until a driver takes over, we put much effort into making sure there is
> > > not even a small window in time where RMRR regions (unity mapped regions
> > > on AMD) are not mapped.
> >
> > Yes, I think the DMA blocking can only start around/after a VFIO type
> > driver has probed() and bound to a device in the group, not much
> > different from today.
>
> Does this mean when a device has a required "RMRR" that requires a unity
> mapping we block assigning those devices to guests? I remember we had some
> restriction but there was a need to go around it at some point in time.
>
> - Either we disallow assigning devices with RMRR
> - Break that unity map when the device is probed and after which any RMRR
> access from device will fault.

We currently disallow assignment of RMRR encumbered devices except for
the known cases of USB and IGD. In the general case, an RMRR imposes
a requirement on the host system to maintain ranges of identity mapping
that is incompatible with userspace ownership of the device and IOVA
address space. AFAICT, nothing changes in the /dev/iommu model that
would make it safe to entrust userspace with RMRR encumbered devices.
Thanks,

Alex