arm64: mm: Pin down ASIDs for sharing mm with devices

hulk inclusion
category: feature
bugzilla: 14369
CVE: NA
-------------------

To enable address space sharing with the IOMMU, introduce mm_context_get()
and mm_context_put(), that pin down a context and ensure that it will keep
its ASID after a rollover.

Pinning is necessary because a device constantly needs a valid ASID,
unlike tasks that only require one when running. Without pinning, we would
need to notify the IOMMU when we're about to use a new ASID for a task,
and it would get complicated when a new task is assigned a shared ASID.
Consider the following scenario with no ASID pinned:

1. Task t1 is running on CPUx with shared ASID (gen=1, asid=1)
2. Task t2 is scheduled on CPUx, gets ASID (1, 2)
3. Task tn is scheduled on CPUy, a rollover occurs, tn gets ASID (2, 1)
   We would now have to immediately generate a new ASID for t1, notify
   the IOMMU, and finally enable task tn. We are holding the lock during
   all that time, since we can't afford having another CPU trigger a
   rollover. The IOMMU issues invalidation commands that can take tens of
   milliseconds.

It gets needlessly complicated. All we wanted to do was schedule task tn,
that has no business with the IOMMU. By letting the IOMMU pin tasks when
needed, we avoid stalling the slow path, and let the pinning fail when
we're out of shareable ASIDs.

After a rollover, the allocator expects at least one ASID to be available
in addition to the reserved ones (one per CPU). So (NR_ASIDS - NR_CPUS -
1) is the maximum number of ASIDs that can be shared with the IOMMU.

Cc: catalin.marinas@arm.com
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Fang Lijun <fanglijun3@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>