On 22/11/17 03:15, Bob Liu wrote:
> Hey Jean,
>
> On 2017/10/6 21:31, Jean-Philippe Brucker wrote:
>> IOMMU drivers need a way to bind Linux processes to devices. This is used
>> for Shared Virtual Memory (SVM), where devices support paging. In that
>> mode, DMA can directly target virtual addresses of a process.
>>
>> Introduce boilerplate code for allocating process structures and binding
>> them to devices. Four operations are added to IOMMU drivers:
>>
>> * process_alloc, process_free: to create an iommu_process structure and
>> perform architecture-specific operations required to grab the process
>> (for instance on ARM SMMU, pin down the CPU ASID). There is a single
>> iommu_process structure per Linux process.
>>
>
> I'm a bit confused here.
> The original meaning of iommu_domain is a virtual addrspace defined by a set of io page table.
> (fix me if I misunderstood).
iommu_domain can also be seen as a logical partition of devices that share
the same address spaces (the concept comes from AMD and Intel IOMMU
domains, I believe). Without PASIDs it was a single address space, with
PASIDs it can have multiple address spaces.
> Then what's the meaning of iommu_domain and iommu_process after introducing iommu_process?
> Could you consider document these concepts?
iommu_process is used to keep track of Linux process address spaces. I'll
rename it to io_mm in next version, to make it clear that it doesn't
represent a Linux task but an mm_struct instead. However the
implementation stays pretty much identical. A domain can be associated to
multiple io_mm, and an io_mm can be associated to multiple domains.
In the IOMMU architectures I know, PASID is implemented like this. You
have the device tables (stream tables on SMMU), pointing to PASID tables
(context descriptor tables on SMMU). In the following diagram,
.->+--------+
/ 0 | |------ io_pgtable
/ +--------+
/ 1 | |------ io_mm->mm X
+--------+ / +--------+
0 | A |-' 2 | |-.
+--------+ +--------+ \
1 | | 3 | | \
+--------+ +--------+ -- io_mm->mm Y
2 | B |--. PASID tables /
+--------+ \ |
3 | B |----+--->+--------+ |
+--------+ / 0 | |- | -- io_pgtable
4 | B |--' +--------+ |
+--------+ 1 | | |
Device tables +--------+ |
2 | |--'
+--------+
3 | |------ io_mm->priv io_pgtable
+--------+
PASID tables
* Device 0 (e.g. PCI 0000:00:00.0) is in domain A.
* Devices 2, 3 and 4 are in domain B.
* Domain A has the top set of PASID tables.
* Domain B has the bottom set of PASID tables.
* Domain A is bound to process address space X.
-> Device 0 can access X with PASID 1.
* Both domains A and B are bound to process address space Y.
-> Devices 0, 2, 3 and 4 can access Y with PASID 2
* PASID 0 is special on Arm SMMU (with S1DSS=0b10). It will always be
reserved for classic DMA map/unmap. Even for hypothetical devices that
don't support non-pasid transactions, I'd like to keep this convention.
It should be quite useful for device drivers to have PASID 0 available
with DMA map/unmap.
* When introducing "private" PASID address spaces (that many are asking
for), which are backed by a set of io-pgtable and map/unmap ops, I
suppose they would reuse the io_mm structure. In this example PASID 3 is
associated to a private address space and not backed by an mm. Since the
PASID space is global, PASID 3 won't be available for any other domain.
Does this clarify the current design, or is it just more confusing?
Thanks,
Jean
