Re: [PATCH v3 3/7] PCI: OF: Allow endpoints to bypass the iommu

[Jean-philippe Brucker <jphilippe.brucker@gmail.com>]

[Replying with my personal address because we're having SMTP issues]

On 15/10/2018 11:52, Michael S. Tsirkin wrote:
> On Fri, Oct 12, 2018 at 02:41:59PM -0500, Bjorn Helgaas wrote:
>> s/iommu/IOMMU/ in subject
>>
>> On Fri, Oct 12, 2018 at 03:59:13PM +0100, Jean-Philippe Brucker wrote:
>>> Using the iommu-map binding, endpoints in a given PCI domain can be
>>> managed by different IOMMUs. Some virtual machines may allow a subset of
>>> endpoints to bypass the IOMMU. In some case the IOMMU itself is presented
>>
>> s/case/cases/
>>
>>> as a PCI endpoint (e.g. AMD IOMMU and virtio-iommu). Currently, when a
>>> PCI root complex has an iommu-map property, the driver requires all
>>> endpoints to be described by the property. Allow the iommu-map property to
>>> have gaps.
>>
>> I'm not an IOMMU or virtio expert, so it's not obvious to me why it is
>> safe to allow devices to bypass the IOMMU.  Does this mean a typo in
>> iommu-map could inadvertently allow devices to bypass it?
> 
> 
> Thinking about this comment, I would like to ask: can't the
> virtio device indicate the ranges in a portable way?
> This would minimize the dependency on dt bindings and ACPI,
> enabling support for systems that have neither but do
> have virtio e.g. through pci.

I thought about adding a PROBE request for this in virtio-iommu, but it
wouldn't be usable by a Linux guest because of a bootstrapping problem.

Early on, Linux needs a description of device dependencies, to determine
in which order to probe them. If the device dependency was described by
virtio-iommu itself, the guest could for example initialize a NIC,
allocate buffers and start DMA on the physical address space (which aborts
if the IOMMU implementation disallows DMA by default), only to find out
once the virtio-iommu module is loaded that it needs to cancel all DMA and
reconfigure the NIC. With a static description such as iommu-map in DT or
ACPI remapping tables, the guest can defer probing of the NIC until the
IOMMU is initialized.

Thanks,
Jean