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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 25 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.

I prefer to avoid a split-mind approach, where some PCI things are
initialized/owned by one component and some others are initialized/owned
by another component. It creates complexity. Of course, we have to face
the reality that the alternatives might be worse, but let's take a look
at the other options first.

How hard would it be to bring the PCI host bridge initialization in Xen,
for example in the case of the Xilinx ZynqMP? Traditionally, PCI host
bridges have not required any initialization on x86. PCI is still new to
the ARM ecosystems. I think it is reasonable to expect that going
forward, as the ARM ecosystem matures, PCI host bridges will require
little to no initialization on ARM too.


> Now regarding the configuration space, I think we can divide in 2 category:
>       - indirect access, the configuration space are multiplexed. An example
> would be the legacy method on x86 (e.g 0xcf8 and 0xcfc). A similar method is
> used for x-gene PCI driver ([1]).
>       - ECAM like access, where each PCI configuration space will have it is
> own address space. I said "ECAM like" because some host bridge will require
> some bits fiddling when accessing register (see thunder-ecam [2])
> 
> There are also host bridges that mix both indirect access and ECAM like access
> depending on the device configuration space accessed (see thunder-pem [3]).
> 
> When using ECAM like host bridge, I don't think it will be an issue to have
> both DOM0 and Xen accessing configuration space at the same time. Although, we
> need to define who is doing what. In general case, DOM0 should not touched an
> assigned PCI device. The only possible interaction would be resetting a device
> (see my answer below).

Even if the hardware allows it, I think it is a bad idea to access the
same hardware component from two different entities simultaneously.

I suggest we trap Dom0 reads/writes to ECAM, and execute them in Xen,
which I think it's what x86 does today.


> When using indirect access, we cannot let DOM0 and Xen accessing any PCI
> configuration space at the same time. So I think we would have to emulate the
> physical host controller.
> 
> Unless we have a big requirement to trap DOM0 access to the configuration
> space, I would only keep the emulation to the strict minimum (e.g for indirect
> access) to avoid ending-up handling all the quirks for ECAM like host bridge.
> 
> If we need to trap the configuration space, I would suggest the following for
> ECAM like host bridge:
>       - For physical host bridge that does not require initialization and is
> nearly ECAM compatible (e.g require register fiddling) => replace by a generic
> host bridge emulation for DOM0

Sounds good.


>       - For physical host bridge that require initialization but is ECAM
> compatible (e.g AFAICT xilinx [4]) => trap the ECAM access but let DOM0
> handling the host bridge initialization

I would consider doing the initialization in Xen. It would simplify the
architecture significantly.


>       - For all other host bridges => I don't know if there are host bridges
> falling under this category. I also don't have any idea how to handle this.

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