Re: [PATCH 1/3] MdeModulePkg/Include: Add IOMMU protocol definition.

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 29 March 2017 at 00:45, Yao, Jiewen <jiewen.yao@intel.com> wrote:
> Agree. That is a good idea.
>
>
>
> I will add that in V2 patch.
>

Hello Jiewen,

As a bit of background, what I have in mind is an enhancement of the
PCI root bridge I/O allocate, map and unmap methods so that situations
that would currently lead to failure or to suboptimal performance are
left for the IOMMU protocol to handle if one is present. Note that
this may imply having IOMMU protocol instances for each PCI root
bridge, and for other masters as well. (For example, AMD Seattle has
separate IOMMUs for PCI and for the networking controllers, which are
wired to the internal interconnect directly)

So in RootBridgeIoMap(), for instance, we have this condition

  PhysicalAddress = (EFI_PHYSICAL_ADDRESS) (UINTN) HostAddress;
  if ((!RootBridge->DmaAbove4G ||
       (Operation != EfiPciOperationBusMasterRead64 &&
        Operation != EfiPciOperationBusMasterWrite64 &&
        Operation != EfiPciOperationBusMasterCommonBuffer64)) &&
      ((PhysicalAddress + *NumberOfBytes) > SIZE_4GB)) {

to decide whether bounce buffering is necessary (or even possible).
The mapping between DeviceAddress and HostAddress could be supplied by
the IOMMU protocol instance, which also means we should reinterpret
DmaAbove4G and other variables related to 32-bit addressing to apply
to the device address and not the physical address.

Similarly, in RootBridgeIoAllocateBuffer(), a failure to allocate
below 4 GB may not be an error if the IOMMU protocol instance can
provide a 32-bit addressable mapping for it.

I am aware that this complicates matters for you, but having IOMMU
support in the generic PCI host bridge driver is extremely useful for
us. I am happy to help out in any way I can.

Thanks,
Ard.