The sva_bind() function allows devices to access process address spaces
using a PASID (aka SSID).

(1) bind() allocates or gets an existing MMU notifier tied to the
    (domain, mm) pair. Each mm gets one PASID.

(2) Any change to the address space calls invalidate_range() which sends
    ATC invalidations (in a subsequent patch).

(3) When the process address space dies, the release() notifier disables
    the CD to allow reclaiming the page tables. Since release() has to
    be light we do not instruct device drivers to stop DMA here, we just
    ignore incoming page faults from this point onwards.

    To avoid any event 0x0a print (C_BAD_CD) we disable translation
    without clearing CD.V. PCIe Translation Requests and Page Requests
    are silently denied. Don't clear the R bit because the S bit can't
    be cleared when STALL_MODEL==0b10 (forced), and clearing R without
    clearing S is useless. Faulting transactions will stall and will be
    aborted by the IOPF handler.

(4) After stopping DMA, the device driver releases the bond by calling
    unbind(). We release the MMU notifier, free the PASID and the bond.

Three structures keep track of bonds:
* arm_smmu_bond: one per {device, mm} pair, the handle returned to the
  device driver for a bind() request.
* arm_smmu_mmu_notifier: one per {domain, mm} pair, deals with ATS/TLB
  invalidations and clearing the context descriptor on mm exit.
* arm_smmu_ctx_desc: one per mm, holds the pinned ASID and pgd.
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20201106155048.997886-4-jean-philippe@linaro.orgSigned-off-by: Will Deacon <will@kernel.org>