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Diffstat (limited to 'Documentation/x86')
| -rw-r--r-- | Documentation/x86/sva.rst | 53 |
1 files changed, 41 insertions, 12 deletions
diff --git a/Documentation/x86/sva.rst b/Documentation/x86/sva.rst index 076efd51ef1f..2e9b8b0f9a0f 100644 --- a/Documentation/x86/sva.rst +++ b/Documentation/x86/sva.rst @@ -104,18 +104,47 @@ 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. + +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 ============= |