Re: [RFC] /dev/ioasid uAPI proposal

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Jun 05, 2021 at 08:22:27AM +0200, Paolo Bonzini wrote:
> On 04/06/21 19:22, Jason Gunthorpe wrote:
> >   4) The KVM interface is the very simple enable/disable WBINVD.
> >      Possessing a FD that can do IOMMU_EXECUTE_WBINVD is required
> >      to enable WBINVD at KVM.
> 
> The KVM interface is the same kvm-vfio device that exists already.  The
> userspace API does not need to change at all: adding one VFIO file
> descriptor with WBINVD enabled to the kvm-vfio device lets the VM use WBINVD
> functionality (see kvm_vfio_update_coherency).

The problem is we are talking about adding a new /dev/ioasid FD and it
won't fit into the existing KVM VFIO FD interface. There are lots of
options here, one is to add new ioctls that specifically use the new
FD, the other is to somehow use VFIO as a proxy to carry things to the
/dev/ioasid FD code.

> Alternatively you can add a KVM_DEV_IOASID_{ADD,DEL} pair of ioctls. But it
> seems useless complication compared to just using what we have now, at least
> while VMs only use IOASIDs via VFIO.

The simplest is KVM_ENABLE_WBINVD(<fd security proof>) and be done
with it.

I don't need to keep track things in KVM, just flip one flag on/off
under user control.

> Either way, there should be no policy attached to the add/delete operations.
> KVM users want to add the VFIO (or IOASID) file descriptors to the device
> independent of WBINVD.  If userspace wants/needs to apply its own policy on
> whether to enable WBINVD or not, they can do it on the VFIO/IOASID side:

Why does KVM need to know abut IOASID's? I don't think it can do
anything with this general information.

> >  1) When the device is attached to the IOASID via VFIO_ATTACH_IOASID
> >     it communicates its no-snoop configuration:
> >      - 0 enable, allow WBINVD
> >      - 1 automatic disable, block WBINVD if the platform
> >        IOMMU can police it (what we do today)
> >      - 2 force disable, do not allow BINVD ever
> 
> Though, like Alex, it's also not clear to me whether force-disable is
> useful.  Instead userspace can query the IOMMU or the device to ensure it's
> not enabled.

"force disable" would be a way for the device to signal to whatever
query you imagine that it is not enabled. Maybe I should have called
it "no-snoop is never used"

Jason