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

[Julien Grall <julien@xxxxxxx>]

Hi Rahul,

On 25/06/2023 15:28, Rahul Singh wrote:
> > On 25 Jun 2023, at 1:56 pm, Julien Grall <julien@xxxxxxx> wrote:
> > 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.
Right, but selecting "pci-passthrough" to be able to use the IOMMU in 
dom0 is confusing. We already support PCI in dom0 and adding the support 
(IOMMU + PCI) has no relation with assigning a device to the guest. IOW 
one may want to use IOMMU in dom0 without assigning devices to the guest.
I think part of the code gated by "pci-passthrough" should also be 
available when the IOMMU is enabled. This would allow users to use IOMMU 
+ PCI in dom0 without any extra patches and/or command line option.
Cheers,

--
Julien Grall