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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 2 Feb 2017, Julien Grall wrote:
> Hi Roger,
> 
> On 01/02/17 10:55, Roger Pau Monné wrote:
> > On Wed, Jan 25, 2017 at 06:53:20PM +0000, Julien Grall wrote:
> > > Hi Stefano,
> > > 
> > > On 24/01/17 20:07, Stefano Stabellini wrote:
> > > > On Tue, 24 Jan 2017, Julien Grall wrote:
> > > > > > > whilst for Device Tree the segment number is not available.
> > > > > > > 
> > > > > > > So Xen needs to rely on DOM0 to discover the host bridges and
> > > > > > > notify Xen
> > > > > > > with all the relevant informations. This will be done via a new
> > > > > > > hypercall
> > > > > > > PHYSDEVOP_pci_host_bridge_add. The layout of the structure will
> > > > > > > be:
> > > > > > 
> > > > > > I understand that the main purpose of this hypercall is to get Xen
> > > > > > and Dom0
> > > > > > to
> > > > > > agree on the segment numbers, but why is it necessary? If Dom0 has
> > > > > > an
> > > > > > emulated contoller like any other guest, do we care what segment
> > > > > > numbers
> > > > > > Dom0 will use?
> > > > > 
> > > > > I was not planning to have a emulated controller for DOM0. The
> > > > > physical one is
> > > > > not necessarily ECAM compliant so we would have to either emulate the
> > > > > physical
> > > > > one (meaning multiple different emulation) or an ECAM compliant.
> > > > > 
> > > > > The latter is not possible because you don't know if there is enough
> > > > > free MMIO
> > > > > space for the emulation.
> > > > > 
> > > > > In the case on ARM, I don't see much the point to emulate the host
> > > > > bridge for
> > > > > DOM0. The only thing we need in Xen is to access the configuration
> > > > > space, we
> > > > > don't have about driving the host bridge. So I would let DOM0 dealing
> > > > > with
> > > > > that.
> > > > > 
> > > > > Also, I don't see any reason for ARM to trap DOM0 configuration space
> > > > > access.
> > > > > The MSI will be configured using the interrupt controller and it is a
> > > > > trusted
> > > > > Domain.
> > > > 
> > > > These last you sentences raise a lot of questions. Maybe I am missing
> > > > something. You might want to clarify the strategy for Dom0 and DomUs,
> > > > and how they differ, in the next version of the doc.
> > > > 
> > > > At some point you wrote "Instantiation of a specific driver for the host
> > > > controller can be easily done if Xen has the information to detect it.
> > > > However, those drivers may require resources described in ASL." Does it
> > > > mean you plan to drive the physical host bridge from Xen and Dom0
> > > > simultaneously?
> > > 
> > > I may miss some bits, so feel free to correct me if I am wrong.
> > > 
> > > My understanding is host bridge can be divided in 2 parts:
> > >   - Initialization of the host bridge
> > >   - Access the configuration space
> > > 
> > > For generic host bridge, the initialization is inexistent. However some
> > > host
> > > bridge (e.g xgene, xilinx) may require some specific setup and also
> > > configuring clocks. Given that Xen only requires to access the
> > > configuration
> > > space, I was thinking to let DOM0 initialization the host bridge. This
> > > would
> > > avoid to import a lot of code in Xen, however this means that we need to
> > > know when the host bridge has been initialized before accessing the
> > > configuration space.
> > 
> > Can the bridge be initialized without Dom0 having access to the ECAM area?
> > If
> > that's possible I would do:
> > 
> > 1. Dom0 initializes the bridge (whatever that involves).
> > 2. Dom0 calls PHYSDEVOP_pci_mmcfg_reserved to register the bridge with Xen:
> >  2.1 Xen scans the bridge and detects the devices.
> >  2.2 Xen maps the ECAM area into Dom0 stage-2 p2m.
> > 3. Dom0 scans the bridge &c (whatever is done on native).
> 
> As Stefano suggested, we should try to initialize the hostbridge in Xen when
> possible. This will avoid a split interaction and our hair too :).
> 
> I am looking at different hostbridge to see how much code would be required in
> Xen to handle them. I think the Xilinx root-complex is an easy one (see the
> discussion in [1]) and it is manageable to get the code in Xen.
> 
> But some are much more complex, for instance the R-Car (see discussion in [2])
> requires clocks, use a specific way to access configuration space and has the
> MSI controller integrated in the root complex. This would require some work
> with DOM0. I will mention the problem in the design document but not going to
> address it at the moment (too complex). Although, we would have to support it
> at some point as the root complex is used in automotive board (see [3]).
> 
> For now I will address:
>       - ECAM compliant/ECAM like root complex
>       - Root complex with simple initialization
> 
> For DT, I would have a fallback on mapping the root complex to DOM0 if we
> don't support it. So DOM0 could still use PCI.
> 
> For ACPI, I am expecting all the platform ECAM compliant or require few
> quirks. So I would mandate the support of the root complex in Xen in order to
> get PCI supported.

Sound good. Ack.

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