Re: [PATCH v4 0/7] SMMU handling for PCIe Passthrough on ARM

[Rahul Singh <Rahul.Singh@xxxxxxx>]

Hi Julien,

> On 25 Jun 2023, at 1:56 pm, Julien Grall <julien@xxxxxxx> wrote:
> 
> Hi,
> 
> On 15/06/2023 22:05, Stewart Hildebrand wrote:
>> On 6/7/23 03:19, Julien Grall wrote:
>>> On 07/06/2023 04:02, Stewart Hildebrand wrote:
>>>> This series introduces SMMU handling for PCIe passthrough on ARM. These 
>>>> patches
>>>> are independent from (and don't depend on) the vPCI reference 
>>>> counting/locking
>>>> work in progress, and should be able to be upstreamed independently.
>>> 
>>> Can you clarify how this code was tested? Does this require code not yet
>>> upstreamed?
>> I'm testing the series standalone (+ config changes) by using a PCI device 
>> in dom0, and also in combination with the vPCI series [3] [4] for 
>> passthrough to a domU.
>> Here are some more details on the test cases I'm using.
> 
> Thanks that's helpful! One comment about the first test case.
> 
>> 1. Using the PCI device in dom0 with the pci-passthrough=yes arg. In this 
>> case a couple of additional config changes [1] [2] are needed to select 
>> CONFIG_HAS_PCI=y, CONFIG_HAS_VPCI=y, and make has_vpci() return true. Aside 
>> from this series itself and the config changes, nothing else 
>> not-yet-upstreamed is required for this test case. It is on my TODO list to 
>> upstream these config changes, which I think will be useful on their own, 
>> not necessarily as part of any other series.
> 
> I find a bit confusing that the IOMMU support for dom0 is gated behind 
> 'pci-passthrough'. I was expecting that the iommu would also be properly 
> configured for PCI if we using 'iommu=yes'.

As per my understanding Xen can configure the iommus for PCI device without 
"pci-passthrough” enabled
if we follow below path:

   1) PCI host bridge is already enumerated and powered on in firmware before 
Xen boot
   2) PCI devices are scanned in Xen.
       
(https://gitlab.com/xen-project/people/bmarquis/xen-arm-poc/-/commit/bce463e1588a45e1bfdf59fc0d5f88b16604e439)
   3) After scanning the PCI devices add PCI devices to iommu ( 
iommu_add_device() )

If PCI host bridge is not enumerated then we need "pci-passthrough” enabled to 
allow Linux to do
enumeration and to inform Xen via PHYSDEVOP_pci_device_add hyper call to add 
PCI devices in Xen
This is implemented as part of PCI passthrough feature.

Regards,
Rahul