Linux Virtualization - Nov 2015 - [PATCH v4 0/6] virtio core DMA API conversion

On Mon, 2015-11-09 at 18:18 -0800, Andy Lutomirski
wrote:
> 
> /* Qumranet donated their vendor ID for devices 0x1000 thru 0x10FF.
> */
> static const struct pci_device_id virtio_pci_id_table[] = {
> ??????? { PCI_DEVICE(0x1af4, PCI_ANY_ID) },
> ??????? { 0 }
> };
> 
> Can we match on that range?
We can, but the problem remains, how do we differenciate an existing
device that does bypass vs. a newer one that needs the IOMMU and thus
doesn't have the new "bypass" property in the device-tree.

Cheers,
Ben.

On Mon, Nov 9, 2015 at 9:28 PM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
> On Mon, 2015-11-09 at 18:18 -0800, Andy Lutomirski wrote:
>>
>> /* Qumranet donated their vendor ID for devices 0x1000 thru 0x10FF.
>> */
>> static const struct pci_device_id virtio_pci_id_table[] = {
>>         { PCI_DEVICE(0x1af4, PCI_ANY_ID) },
>>         { 0 }
>> };
>>
>> Can we match on that range?
>
> We can, but the problem remains, how do we differenciate an existing
> device that does bypass vs. a newer one that needs the IOMMU and thus
> doesn't have the new "bypass" property in the device-tree.
>
We could do it the other way around: on powerpc, if a PCI device is in
that range and doesn't have the "bypass" property at all, then
it's
assumed to bypass the IOMMU.  This means that everything that
currently works continues working.  If someone builds a physical
virtio device or uses another system in PCIe target mode speaking
virtio, then it won't work until they upgrade their firmware to set
bypass=0.  Meanwhile everyone using hypothetical new QEMU also gets
bypass=0 and no ambiguity.

vfio will presumably notice the bypass and correctly refuse to map any
current virtio devices.

Would that work?

--Andy

On Mon, 2015-11-09 at 21:35 -0800, Andy Lutomirski
wrote:
> 
> We could do it the other way around: on powerpc, if a PCI device is in
> that range and doesn't have the "bypass" property at all,
then it's
> assumed to bypass the IOMMU.??This means that everything that
> currently works continues working.??If someone builds a physical
> virtio device or uses another system in PCIe target mode speaking
> virtio, then it won't work until they upgrade their firmware to set
> bypass=0.??Meanwhile everyone using hypothetical new QEMU also gets
> bypass=0 and no ambiguity.
>
> vfio will presumably notice the bypass and correctly refuse to map any
> current virtio devices.
> 
> Would that work?
That would be extremely strange from a platform perspective. Any device
in that vendor/device range would bypass the iommu unless some new
property "actually-works-like-a-real-pci-device" happens to exist in
the device-tree, which we would then need to define somewhere and
handle accross at least 3 different platforms who get their device-tree 
from widly different places.

Also if tomorrow I create a PCI device that implements virtio-net and
put it in a machine running IBM proprietary firmware (or Apple's or
Sun's), it won't have that property...

This is not hypothetical. People are using virtio to do point-to-point
communication between machines via PCIe today.

Cheers,
Ben.