Re: [PATCH v5.99.1 RFC 1/4] xen/arm: Duplicate gic-v2.c file to support hip04 platform version

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-02-25 at 16:59 +0000, Ian Campbell wrote:
> > I think we should disable the build of all drivers in Xen by default,
> > except for the ARM standard compliant ones (for aarch64 the SBSA is a
> > nice summary of what is considered compliant), to keep the size of the
> > binary small.
> 
> I don't think the SBSA is necessarily the best reference here, since it
> only defines what is standardised within the scope of "server
> systems" (whatever you take that to mean) and there are things which do
> not fall under that umbrella.
> 
> That said I'm not sure what better reference there is.
> 
> Maybe even on non-server systems the set of hardware which Xen has to
> drive itself is limited to things which are covered by the SBSA in
> practice, by the nature of the fact that the majority of the wacky stuff
> is driven from hardware domains.
> 
> So maybe SBSA is OK then...

Having thought about this overnight I'm now not so sure of this again, I
don't think SBSA is the definition we want.

Lets take a step back...

The set of hardware which Xen needs to be able to drive (rather than
give to a hardware domain) is:

      * CPUs
      * Host interrupt controllers
      * Host timers
      * A UART
      * Second-state MMUs
      * Second-stage SMMUs/IOMMU (First-stage used by domains)
      * PCI host bridges (in the near future)

(did I forget any?)

NB: I'm only considering host level stuff here. Our virtualised hardware
as exposed to the guest is well defined right now and any conversation
about deviating from the set of hardware (e.g. providing a guest view to
a non-GIC compliant virtual interrupt controller) would be part of a
separate larger conversation about "hvm" style guests (and I'd, as you
might imagine, be very reluctant to code to Xen itself to support
non-standard vGICs in particular).

>From a "what does 'standards compliant' mean" PoV we have:

CPUs:

        Specified in the ARM ARM (v7=ARM DDI 0406, v8=ARM DDI 0487).
        
        Uncontroversial, I hope!
        
Host interrupt controllers:

        Defined in "ARM Generic Interrupt Controller Architecture
        Specification" (v2=ARM IHI 0048B, v3=???).
        
        Referenced from ARMv8 ARM (but not required AFAICT) but
        nonetheless this is what we consider when we talk about the
        "standard interrupt controller".
        
Host timers:

        Defined in the ARMv8 ARM "Generic Timers" chapter.
        
        Defined as an extension to ARMv7 (don't have doc ref handy). For
        our purposes such extensions are considered standardized[*].

UARTS:

        SBSA defines some (pl011 only?), but there are lots, including
        8250-alike ones (ns16550 etc) which is a well established
        standard (from x86).
        
        Given that UART drivers are generally small and pretty trivial I
        think we can tolerate "non-standard" (i.e. non-SBSA, non-8250)
        ones, so long as they are able to support our vuart interface.
        
        I think the non-{pl011,8250} ones should be subject to non-core
        (i.e. community) maintenance as I mentioned previously, i.e
        should be listed in MAINTAINERS other than under the core ARM
        entry. If we decide to go ahead with this approach I'll ask the
        appropriate people to update MAINTAINERS.

Second stage MMU:

        Defined in the ARMv8 ARM, or the ARMv7 LPAE extensions[*, **].
        
Second stage SMMU/IOMMU:

        Defined in "ARM System Memory Management Unit Architecture
        Specification" (ARM IHI 0062D?)
        
PCI host bridges:

        We don't actually (properly) support PCI yet, but see my mails
        in the "Enhance platform support for PCI" thread this morning,
        for what we think the general shape might be taking.
        
        The meaning of the PCI CFG (CAM, ECAM etc), MMIO and IO (MMIO
        mapped) regions are, I think, all pretty well defined by the
        (various?) PCI spec(s).
        
        What's not quite so clear cut is the discovery of where the
        controllers actually live (based on the fact that
        Documentation/devicetree/bindings/pci/ has more than one entry
        in it...).
        
        SBSA doesn't really cover this either, it says "The base address
        of each ECAM region within the system memory map is
        IMPLEMENTATION DEFINED and is expected to be discoverable from
        system firmware data."
        
        However I think it will be the case that most "pci host bridge
        drivers" for Xen will end up being fairly trivial affairs, i.e.
        parse the DT, perhaps a little bit of basic setup and register
        the resulting regions with the PCI core and perhaps provide some
        simple accessors.
        
        So, I think we can say that for PCI controllers which export a
        set of PCI standard CFG, MMIO, IO space regions (all as MMIO
        mapped regions) and just require a specific driver for discovery
        of the host bridge and small amounts of start of day setup
        should be considered "standard" for our purposes.

So, based on the above I think we overall have a pretty good idea what
"standardized" means for the hardware components which we care about.

I think the above is a workable definition of what it is reasonable to
expect the core Xen/ARM maintainers to look after (with that particular
hat on) vs. what it should be expected for interested members of the
community to step up and maintain (where the people who happen to be
core Xen/ARM maintainers may occasionally chose to have such a hat too.)

I think it also provides a workable definition for the line for
Stefano's HAS_NON_STANDARD_DRIVERS idea as well, if we want to go down
that path.

Ian.

[*] For ARMv7 I think we can consider things like the Generic Timers and
LPAE extension specs to be morally equivalent to the bits of the ARMv8
ARM spec, i.e. if it was merged into the ARMv8 ARM then we treat it as
part of the ARMv7 ARM from a "is it standardized" PoV.

[**] The LPAE extensions include/are mixed with the hyp mode page table
format, so we pretty certainly need it.