Re: [Xen-devel] [RFC] ARM PCI Passthrough design document

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Roger,

On 30/05/17 08:53, Roger Pau Monné wrote:
> On Mon, May 29, 2017 at 07:14:55PM +0100, Julien Grall wrote:
> > On 05/29/2017 03:30 AM, Manish Jaggi wrote:
> > > On 5/26/2017 10:44 PM, Julien Grall wrote:
> [...]
> > > > ## Discovering and registering PCI devices
> > > >
> > > > The hardware domain will scan the host bridge to find the list of PCI
> > > > devices
> > > > available and then report it to Xen using the existing hypercall
> > > > PHYSDEV_pci_device_add:
> > > >
> > > > #define XEN_PCI_DEV_EXTFN   0x1
> > > > #define XEN_PCI_DEV_VIRTFN  0x2
> > > > #define XEN_PCI_DEV_PXM     0x3
> > > >
> > > > struct physdev_pci_device_add {
> > > >      /* IN */
> > > >      uint16_t    seg;
> > > >      uint8_t     bus;
> > > >      uint8_t     devfn;
> > > >      uint32_t    flags;
> > > >      struct {
> > > >          uint8_t bus;
> > > >          uint8_t devfn;
> > > >      } physfn;
> > > >      /*
> > > >       * Optional parameters array.
> > > >       * First element ([0]) is PXM domain associated with the device (if
> > > >       * XEN_PCI_DEV_PXM is set)
> > > >       */
> > > >      uint32_t optarr[0];
> > > > }
> > > For mapping the MMIO space of the device in Stage2, we need to add
> > > support in Xen / via a map hypercall in linux/drivers/xen/pci.c
> > Mapping MMIO space in stage-2 is not PCI specific and already addressed in
> > Xen 4.9 (see commit 80f9c31 "xen/arm: acpi: Map MMIO on fault in stage-2
> > page table for the hardware domain"). So I don't understand why we should
> > care about that here...
> I'm not sure what Manish means, but you should map the BARs of the
> device when adding it to a domain.
This could be done when configuring the BARs. Today for DOM0, we rely 
either on trapping or XENMEM_add_to_add_physmap.
But I still don't understand why it matters so much in the design 
document. This is really an implementation details.
> Doing mapping on faults will work
> with CPU accesses, but it's not going to work with SMMU faults, those are
> asynchronous, and I don't think you can guarantee that the CPU is
> always going to access the BARs before doing any DMA transactions to
> them.
Why would you do DMA using BARs? I thought DMA was only to/from memory?

> Note that Xen can also scan the bridge by itself and add the devices,
> I'm not sure you need the PHYSDEV_pci_device_add hypercall.
This should work today without any knowledge of PCIs in Xen. I am not 
aware of any failures with the current approach implemented. If you 
think it does not work, then please give a concrete example.
Cheers,

--
Julien Grall

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