[PATCH v2 11/11] docs: x86: Change documentation for SVA (Shared Virtual Addressing)

[Fenghua Yu <fenghua.yu@intel.com>]

Since allocating, freeing and fixing up PASID are changed, the
documentation is updated to reflect the changes.

Originally-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
---
v2:
- Update life cycle info (Tony and Thomas).
- Update initial PASID value to INVALID_IOASID on fork().

 Documentation/x86/sva.rst | 58 +++++++++++++++++++++++++++++++--------
 1 file changed, 46 insertions(+), 12 deletions(-)

diff --git a/Documentation/x86/sva.rst b/Documentation/x86/sva.rst
index 076efd51ef1f..92341f26e525 100644
--- a/Documentation/x86/sva.rst
+++ b/Documentation/x86/sva.rst
@@ -104,18 +104,52 @@ The MSR must be configured on each logical CPU before any application
 thread can interact with a device. Threads that belong to the same
 process share the same page tables, thus the same MSR value.
 
-PASID is cleared when a process is created. The PASID allocation and MSR
-programming may occur long after a process and its threads have been created.
-One thread must call iommu_sva_bind_device() to allocate the PASID for the
-process. If a thread uses ENQCMD without the MSR first being populated, a #GP
-will be raised. The kernel will update the PASID MSR with the PASID for all
-threads in the process. A single process PASID can be used simultaneously
-with multiple devices since they all share the same address space.
-
-One thread can call iommu_sva_unbind_device() to free the allocated PASID.
-The kernel will clear the PASID MSR for all threads belonging to the process.
-
-New threads inherit the MSR value from the parent.
+PASID Life Cycle Management
+==========================
+
+PASID is initialized as INVALID_IOASID (-1) when a process is created.
+
+Only processes that access SVA-capable devices need to have a PASID
+allocated. This allocation happens when a process opens/binds an SVA-capable
+device but finds no PASID for this process. Subsequent binds of the same, or
+other devices will share the same PASID.
+
+Although the PASID is allocated to the process by opening a device,
+it is not active in any of the threads of that process. It's loaded to the
+IA32_PASID MSR lazily when a thread tries to submit a work descriptor
+to a device using the ENQCMD.
+
+That first access will trigger a #GP fault because the IA32_PASID MSR
+has not been initialized with the PASID value assigned to the process
+when the device was opened. The Linux #GP handler notes that a PASID has
+been allocated for the process, and so initializes the IA32_PASID MSR
+and returns so that the ENQCMD instruction is re-executed.
+
+On fork(2) or exec(2) the PASID is removed from the process as it no
+longer has the same address space that it had when the device was opened.
+
+On clone(2) the new task shares the same address space, so will be
+able to use the PASID allocated to the process. The IA32_PASID is not
+preemptively initialized as the PASID value might not be allocated yet or
+the kernel does not know whether this thread is going to access the device
+and the cleared IA32_PASID MSR reduces context switch overhead by xstate
+init optimization. Since #GP faults have to be handled on any threads that
+were created before the PASID was assigned to the mm of the process, newly
+created threads might as well be treated in a consistent way.
+
+Due to complexity of freeing the PASID and clearing all IA32_PASID MSRs in
+all threads in unbind, free the PASID lazily only on mm exit. Track the
+PASID's reference count in the following way:
+
+- Initialize the PASID's reference to 1 when the PASID is first allocated
+  to the mm. The reference is held for the rest life time of the mm until
+  it's dropped to 0 and the PASID is freed on mm exit.
+
+If a process does a close(2) of the device file descriptor and munmap(2)
+of the device MMIO portal, then the driver will unbind the device. The
+PASID is still marked VALID in the PASID_MSR for any threads in the
+process that accessed the device. But this is harmless as without the
+MMIO portal they cannot submit new work to the device.
 
 Relationships
 =============
-- 
2.34.1