iommu_mode transitions

[just-for-fun-too]

Hi

In Section 6.5 the following statement is made:

The IOMMU behavior of writing iommu_mode to 1LVL, 2LVL, or 3LVL, when the previous value of
the iommu_mode is not Off or Bare is UNSPECIFIED. To change DDT levels, the IOMMU must first
be transitioned to Bare or Off state. The behavior resulting from changing the iommu_mode to
Bare when the previous value of the iommu_mode was not Off is UNSPECIFIED.

1. In the first sentence it states that a transition to 1LVL, 2LVL, or 3LVL has to be done from either the state being Off or Bare otherwise it is UNSPECIFIED.
   Bare -> 1LVL, 2LVL, 3LVL
   Off -> 1LVL, 2LVL, 3LVL
   xLVL -> yLVL (UNSPECIFIED)
2. In the second sentence it states that to change DDT levels (meaning to change to 1LVL, 2LVL, or 3LVL) you must transition to Off or Bare.
   xLVL -> Off
   xLVL -> Bare (OK)
   Off or Bare -> yLVL
3. In the third sentence it states that to go to Bare you have to go to Off otherwise the behavior is UNSPECIFIED.
   Off -> Bare
   xLVL -> Bare (UNSPECIFIED)

It seems the second and third sentences are inconsistent if you look at the bold text. Please clarify. Thanks!

[ved-rivos]

In your second point, the xLVL -> Bare is unspecified because of rule 3.

The specified behaviors are for following transitions:

• Off -> *LVL or Bare
• *LVL -> Off
• Bare -> Off or *LVL

The following transitions have unspecified behavior:

• *LVL -> Bare
• xLVL -> yLVL

To change levels, one would need to first transition to Off. From Off one can transition either directly to xLVL or first transition to Bare and then transition to xLVL.

[just-for-fun-too]

Thanks Ved!