Re: [PATCH v5 03/18] pci: isolated address space for PCI bus

[Alex Williamson]

On Wed, 2 Feb 2022 09:30:42 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> On Tue, 1 Feb 2022 at 23:51, Alex Williamson <alex.williamson@redhat.com> 
> wrote:
> >
> > On Tue, 1 Feb 2022 21:24:08 +0000
> > Jag Raman <jag.raman@oracle.com> wrote:  
> > > The PCIBus data structure already has address_space_mem and
> > > address_space_io to contain the BAR regions of devices attached
> > > to it. I understand that these two PCIBus members form the
> > > PCI address space.  
> >
> > These are the CPU address spaces.  When there's no IOMMU, the PCI bus is
> > identity mapped to the CPU address space.  When there is an IOMMU, the
> > device address space is determined by the granularity of the IOMMU and
> > may be entirely separate from address_space_mem.  
> 
> Note that those fields in PCIBus are just whatever MemoryRegions
> the pci controller model passed in to the call to pci_root_bus_init()
> or equivalent. They may or may not be specifically the CPU's view
> of anything. (For instance on the versatilepb board, the PCI controller
> is visible to the CPU via several MMIO "windows" at known addresses,
> which let the CPU access into the PCI address space at a programmable
> offset. We model that by creating a couple of container MRs which
> we pass to pci_root_bus_init() to be the PCI memory and IO spaces,
> and then using alias MRs to provide the view into those at the
> guest-programmed offset. The CPU sees those windows, and doesn't
> have direct access to the whole PCIBus::address_space_mem.)
> I guess you could say they're the PCI controller's view of the PCI
> address space ?

Sure, that's fair.

> We have a tendency to be a bit sloppy with use of AddressSpaces
> within QEMU where it happens that the view of the world that a
> DMA-capable device matches that of the CPU, but conceptually
> they can definitely be different, especially in the non-x86 world.
> (Linux also confuses matters here by preferring to program a 1:1
> mapping even if the hardware is more flexible and can do other things.
> The model of the h/w in QEMU should support the other cases too, not
> just 1:1.)

Right, this is why I prefer to look at the device address space as
simply an IOVA.  The IOVA might be a direct physical address or
coincidental identity mapped physical address via an IOMMU, but none of
that should be the concern of the device.
 
> > I/O port space is always the identity mapped CPU address space unless
> > sparse translations are used to create multiple I/O port spaces (not
> > implemented).  I/O port space is only accessed by the CPU, there are no
> > device initiated I/O port transactions, so the address space relative
> > to the device is irrelevant.  
> 
> Does the PCI spec actually forbid any master except the CPU from
> issuing I/O port transactions, or is it just that in practice nobody
> makes a PCI device that does weird stuff like that ?

As realized in reply to MST, more the latter.  Not used, no point to
enabling, no means to enable depending on the physical IOMMU
implementation.  Thanks,

Alex