IOMMU (was: USB HID driver fails to recognize input devices)

[Julian Stecklina]

Hey,

there is a system call that assigns PCI devices to PDs. A PD proves that is allowed to drive a specific device by presenting the respective PCI MMCONFIG page to the kernel. In addition, memory supposed to be visible to devices must be mapped with the D bit set.

HTH, Julian

Norman Feske <norman.feske at ...1...> wrote:

>Hi Udo,
>
>thanks for your very helpful explanation!
>
>I am just wondering: If DMA addresses are host-virtual addresses and
>there are multiple processes running, how does the IOMMU know which
>virt-to-phys translation to use? I can loosely remember that you once
>told me that devices must be associated with PDs. Is this correct? If
>so, how does a driver PD express to the hypervisor that it deals with
>a certain device?
>
>> With the IOMMU active, a user-level device driver must specify
>> which memory regions of its PD are DMA-able. This is done by
>> setting the D-bit in the Delegate Transfer Item. All memory
>> mappings where the D-bit is not set will not be DMA-able (unless
>> the IOMMU is inactive). My recommendation is that device drivers
>> map their own code and private non-DMA-able and only allow DMA
>> transfers to I/O buffer regions.
>
>This seems to fit quite nicely with the recent addition of a facility
>to explicitly allocate DMA buffers via core's RAM session interface:
>
>
>
>https://github.com/genodelabs/genode/commit/288fd4e56e636a0b3eb193353cf8028614a018b7
>
>Currently, this function is meaningful only on ARM on Fiasco.OC. But
>it looks like a good way to handle the D-bit on NOVA without the need
>of any special precautions at the driver side. Because core is the
>pager of the driver, it could always map DMA buffers (and only those)
>with the D-bit set.
>
>> Also, when the IOMMU is active, the addresses programmed into DMA
>> transfers must be host-virtual addresses. This alleviates device
>> drivers from having to know physical memory addresses. They can DMA
>> into their virtual address space. In contrast, when the IOMMU is
>> inactive, the addresses in DMA transfers must be host-physical
>> addresses.
>
>Is the use of virtual addresses mandatory? If so, the driver must be
>aware of the presence of the IOMMU, doesn't it? It would be nice to
>find a way of using the IOMMU in a way that is transparent to the
>driver.
>
>Cheers
>Norman

-- 
Sent from my phone. Please excuse my brevity.