Re: [Qemu-devel] Re: [RFC PATCH 4/7] ide: IOMMU support

[Eduard - Gabriel Munteanu]

On Thu, Jul 15, 2010 at 07:45:06AM -0500, Anthony Liguori wrote:
> 
> No.  PCI devices should never call cpu_physical_memory*.
> 
> PCI devices should call pci_memory*.
> 
> ISA devices should call isa_memory*.
> 
> All device memory accesses should go through their respective buses.  
> There can be multiple IOMMUs at different levels of the device 
> hierarchy.  If you don't provide bus-level memory access functions that 
> chain through the hierarchy, it's extremely difficult to implement all 
> the necessary hooks to perform the translations at different places.
> 
> Regards,
> 
> Anthony Liguori
> 

I liked Paul's initial approach more, at least if I understood him
correctly. Basically I'm suggesting a single memory_* function that
simply asks the bus for I/O and translation. Say you have something like
this:

+ Bus 1
|
---- Memory 1
|
---+ Bus 2 bridge
   |
   ---- Memory 2
   |
   ---+ Bus 3 bridge
      |
      ---- Device

Say Device wants to write to memory. If we have the DeviceState we
needn't concern whether this is a BusOneDevice or BusTwoDevice from
device code itself. We would just call

memory_rw(dev_state, addr, buf, size, is_write);

which simply recurses through DeviceState's and BusState's through their
parent pointers. The actual bus can set up those to provide
identification information and perhaps hooks for translation and access
checking. So memory_rw() looks like this (pseudocode):

static void memory_rw(DeviceState *dev,
                      target_phys_addr_t addr,
                      uint8_t *buf,
                      int size,
                      int is_write)
{
        BusState *bus = get_parent_bus_of_dev(dev);
        DeviceState *pdev = get_parent_dev(dev);
        target_phys_addr_t taddr;

        if (!bus) {
                /* This shouldn't happen. */
                assert(0);
        }

        if (bus->responsible_for(addr)) {
                raw_physical_memory_rw(addr, buf, size, is_write);
                return;
        }

        taddr = bus->translate(dev, addr);
        memory_rw(pdev, taddr, buf, size, is_write);
}

If we do this, it seems there's no need to provide separate
functions. The actual buses must instead initialize those hooks
properly. Translation here is something inherent to the bus, that
handles arbitration between possibly multiple IOMMUs. Our memory would
normally reside on / belong to the top-level bus.

What do you think? (Naming could be better though.)


        Eduard