Re: [Qemu-devel] [PATCH v2] pci: Introduce helper to retrieve a PCI device's DMA address space

[Alexey Kardashevskiy]

On 08/09/2013 09:07 PM, Paolo Bonzini wrote:
> Il 09/08/2013 12:58, Alexey Kardashevskiy ha scritto:
>> On 08/09/2013 08:19 PM, Paolo Bonzini wrote:
>>> Il 09/08/2013 12:13, Alexey Kardashevskiy ha scritto:
>>>> On 08/09/2013 07:53 PM, Paolo Bonzini wrote:
>>>>> Il 09/08/2013 11:48, Alexey Kardashevskiy ha scritto:
>>>>>> On 08/09/2013 07:40 PM, Paolo Bonzini wrote:
>>>>>>> Il 09/08/2013 10:49, Alexey Kardashevskiy ha scritto:
>>>>>>>> A PCI device's DMA address space (possibly an IOMMU) is returned by a
>>>>>>>> method on the PCIBus.  At the moment that only has one caller, so the
>>>>>>>> method is simply open coded.  We'll need another caller for VFIO, so
>>>>>>>> this patch introduces a helper/wrapper function.
>>>>>>>>
>>>>>>>> Signed-off-by: David Gibson <address@hidden>
>>>>>>>> [aik: added inheritance from parent if iommu is not set for the 
>>>>>>>> current bus]
>>>>>>>> Signed-off-by: Alexey Kardashevskiy <address@hidden>
>>>>>>>>
>>>>>>>> ---
>>>>>>>> Changes:
>>>>>>>> v2:
>>>>>>>> * added inheritance, needed for a pci-bridge on spapr-ppc64
>>>>>>>> * pci_iommu_as renamed to pci_device_iommu_address_space
>>>>>>>> ---
>>>>>>>>  hw/pci/pci.c         | 22 ++++++++++++++++------
>>>>>>>>  include/hw/pci/pci.h |  1 +
>>>>>>>>  2 files changed, 17 insertions(+), 6 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
>>>>>>>> index 4c004f5..0072b54 100644
>>>>>>>> --- a/hw/pci/pci.c
>>>>>>>> +++ b/hw/pci/pci.c
>>>>>>>> @@ -812,12 +812,7 @@ static PCIDevice 
>>>>>>>> *do_pci_register_device(PCIDevice *pci_dev, PCIBus *bus,
>>>>>>>>      }
>>>>>>>>  
>>>>>>>>      pci_dev->bus = bus;
>>>>>>>> -    if (bus->iommu_fn) {
>>>>>>>> -        dma_as = bus->iommu_fn(bus, bus->iommu_opaque, devfn);
>>>>>>>> -    } else {
>>>>>>>> -        /* FIXME: inherit memory region from bus creator */
>>>>>>>> -        dma_as = &address_space_memory;
>>>>>>>> -    }
>>>>>>>> +    dma_as = pci_device_iommu_address_space(pci_dev);
>>>>>>>>  
>>>>>>>>      memory_region_init_alias(&pci_dev->bus_master_enable_region,
>>>>>>>>                               OBJECT(pci_dev), "bus master",
>>>>>>>> @@ -2239,6 +2234,21 @@ static void pci_device_class_init(ObjectClass 
>>>>>>>> *klass, void *data)
>>>>>>>>      k->props = pci_props;
>>>>>>>>  }
>>>>>>>>  
>>>>>>>> +AddressSpace *pci_device_iommu_address_space(PCIDevice *dev)
>>>>>>>> +{
>>>>>>>> +    PCIBus *bus = PCI_BUS(dev->bus);
>>>>>>>> +
>>>>>>>> +    if (bus->iommu_fn) {
>>>>>>>> +        return bus->iommu_fn(bus, bus->iommu_opaque, dev->devfn);
>>>>>>>> +    }
>>>>>>>> +
>>>>>>>> +    if (bus->parent_dev) {
>>>>>>>> +        return pci_device_iommu_address_space(bus->parent_dev);
>>>>>>>> +    }
>>>>>>>
>>>>>>> No, this would fail if bus->parent_dev is not NULL but not a PCI device
>>>>>>> either.
>>>>>>
>>>>>> parent_dev is of the PCIDevice* type, how can it be not a PCI device? :-/
>>>>>
>>>>> Doh, I misread the code, I thought it was the "parent" field in
>>>>> BusState.  Why do we have parent_dev at all?
>>>>
>>>> The code is too old? Don't know.
>>>>
>>>>
>>>>>>> You can use object_dynamic_cast to convert the parent_dev to
>>>>>>> PCIDevice, and if the cast succeeds you call the new function.
>>>>>>>
>>>>>>> Perhaps you could make the new function take a PCIBus instead.
>>>>>>> Accessing the PCIDevice's IOMMU address space (as opposed to the
>>>>>>> bus-master address space) doesn't make much sense, VFIO is really a
>>>>>>> special case.  Putting the new API on the bus side instead looks better.
>>>>>>>
>>>>>>> (BTW, do you need to enable bus-master DMA on PCI bridges, to do DMA for
>>>>>>> devices sitting on the secondary bus?)
>>>>>>
>>>>>> It happens naturally I guess when linux enables devices.
>>>>>
>>>>> Yes, but then using the IOMMU address space would be wrong; you would
>>>>> have to use the bus-master address space as a base for the child's
>>>>> bus-master address space.
>>>>
>>>>
>>>> Like this? Works too.
>>>>
>>>> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
>>>> index 8bdcedc..a4c70e6 100644
>>>> --- a/hw/pci/pci.c
>>>> +++ b/hw/pci/pci.c
>>>> @@ -2247,23 +2247,23 @@ AddressSpace
>>>> *pci_device_iommu_address_space(PCIDevice *dev)
>>>>  {
>>>>      PCIBus *bus = PCI_BUS(dev->bus);
>>>>
>>>>      if (bus->iommu_fn) {
>>>>          return bus->iommu_fn(bus, bus->iommu_opaque, dev->devfn);
>>>>      }
>>>>
>>>>      if (bus->parent_dev) {
>>>>          return pci_device_iommu_address_space(bus->parent_dev);
>>>>      }
>>>>
>>>> -    return &address_space_memory;
>>>> +    return &dev->bus_master_as;
>>>>  }
>>>
>>> I was thinking more like this:
>>>
>>>      if (bus->parent_dev) {
>>> -        return pci_device_iommu_address_space(bus->parent_dev);
>>> +        /* Take parent device's bus-master enable bit into account.  */
>>> +        return pci_get_address_space(bus->parent_dev);
>>>      }
>>>
>>> +    /* Not a secondary bus and no IOMMU.  Use system memory.  */
>>>      return &address_space_memory;
>>
>> Ok.
>>
>> Oh. BTW. This "bus master" thing now breaks VFIO's check for all devices
>> being in the same address space as every single device has its own "bus
>> master" AddressSpace.
> 
> Yes, fixing that check is another good use of the new API you introduced.
> 
> But after Ben's answer, I guess the above change is not really needed.
> It would add complication for VFIO, too.  Proper emulation would require
> QEMU to trap writes to the device's bus-master bit.  QEMU would have to
> take of the value that the guest writes, AND it with the bus-master
> bit(s) of all bridges between the host bridge and the VFIO device, and
> write the result to the passed-through device.  This is because the
> bridges are emulated, and do not exist in real hardware.


So does this mean that we go with the original patch and ignore bus master
address space here?


I guess I could overcome that VFIO check by comparing not just AddressSpace
but AddressSpace->root if AddressSpace is different but it does not make a
lot of sense.

> 
>>>>> Also, we would have to fix the x86 firmware.
>>
>> btw what would it mean? :)
> 
> Firmware would have to look for bridges and enable bus master DMA on
> them.  Otherwise, for example a SCSI controller below a bridge (not
> virtio-scsi) would fail to boot.
> 
> Paolo
> 


-- 
Alexey