Re: [PATCH v4 29/32] vfio-pci/zdev: add DTSM to clp group capability

[Matthew Rosato]

On 3/14/22 5:49 PM, Jason Gunthorpe wrote:
> On Mon, Mar 14, 2022 at 03:44:48PM -0400, Matthew Rosato wrote:
> > The DTSM, or designation type supported mask, indicates what IOAT formats
> > are available to the guest.  For an interpreted device, userspace will not
> > know what format(s) the IOAT assist supports, so pass it via the
> > capability chain.  Since the value belongs to the Query PCI Function Group
> > clp, let's extend the existing capability with a new version.
>
> Why is this on the VFIO device?

Current vfio_pci_zdev support adds a series of capability chains to the 
VFIO_DEVICE_GET_INFO ioctl.  These capability chains are all related to 
output values associated with what are basically s390x query instructions.

The capability chain being modified by this patch is used to populate a 
response to the 'query this zPCI group' instruction.

> Maybe I don't quite understand it right, but the IOAT is the
> 'userspace page table'?

IOAT = I/O Address Translation tables;  the head of which is called the 
IOTA (translation anchor).  But yes, this would generally refer to the 
guest DMA tables.

Specifically here we are only talking about the DTSM which is the 
formats that the guest is allowed to use for their address translation 
tables, because the hardware (or in our case the intermediary kvm iommu) 
can only operate on certain formats.

> That is something that should be modeled as a nested iommu domain.
>
> Querying the formats and any control logic for this should be on the
> iommu side not built into VFIO.

I agree that the DTSM is really controlled by what the IOMMU domain can 
support (e.g. what guest table formats it can actually operate on) and 
so the DTSM value should come from there vs out of KVM; but is there 
harm in including the query response data here along with the rest of 
the response information for 'query this zPCI group'?