RE: [RFC] /dev/ioasid uAPI proposal

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, June 3, 2021 8:41 PM
> 
> > When discussing I/O page fault support in another thread, the consensus
> > is that an device handle will be registered (by user) or allocated (return
> > to user) in /dev/ioasid when binding the device to ioasid fd. From this
> > angle we can register {ioasid_fd, device_handle} to KVM and then call
> > something like ioasidfd_device_is_coherent() to get the property.
> > Anyway the coherency is a per-device property which is not changed
> > by how many I/O page tables are attached to it.
> 
> It is not device specific, it is driver specific
> 
> As I said before, the question is if the IOASID itself can enforce
> snoop, or not. AND if the device will issue no-snoop or not.

Sure. My earlier comment was based on the assumption that all IOASIDs
attached to a device should inherit the same snoop/no-snoop fact. But
looks it doesn't prevent a device driver from setting PTE_SNP only for
selected I/O page tables, according to whether isoch agents are involved.

An user space driver could figure out per-IOASID requirements itself.

A guest device driver can indirectly convey this information through 
vIOMMU.

Registering {IOASID_FD, IOASID} to KVM has another merit, as we also
need it to update CPU PASID mapping for ENQCMD. We can define
one interface for both requirements. 😊

Thanks
Kevin