Re: Plan for /dev/ioasid RFC v2

Raj, Ashok
2021-06-18T15:37:42Z

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.

Cheers,
Ashok