openvmm: start each PIT low/high count with its low byte

doc/openvmm-upstream-roadmap.md

@@ -10,9 +10,10 @@
   summary, and commit mapping below cover positions 001-131, the PR09 fixups at positions
   241-243, the PR13 fixup at position 247, the PR18 fixup at position 248, the PR20 fixup at
   position 249, and the PR19 fixup at position 250.
-- NVX's pinned OpenVMM revision: `c2c21ddb25822e13bd1cf1b33ed693545cc7bbb5`, position 262, the
-  head of microsoft/openvmm#5056, whose commit follows position 261, `c72fc56a3`, the head of
-  `feature/microvm/dev`, which microsoft/openvmm#5055 merged with positions 259 and 260 after
+- NVX's pinned OpenVMM revision: `5d656d854a8daca0f23833666b0b71229fe2ab73`, position 263, the
+  head of microsoft/openvmm#5061, whose commit follows position 262, `c2c21ddb2`, the head of
+  `feature/microvm/dev`, which microsoft/openvmm#5056 merged after position 261, `c72fc56a3`,
+  which microsoft/openvmm#5055 merged with positions 259 and 260 after
   position 258, `024ecee62`, which microsoft/openvmm#5053 merged with positions 256 and 257 after
   position 255, `90f87319d`, which microsoft/openvmm#5052 merged with position 254 after position
   253, `3cd9f149b`, which microsoft/openvmm#5046 merged after position 252, `9f10ce7a8`,
@@ -47,8 +48,8 @@
   position 221 into positions 035 and 036, and combined v5.1 positions 222 and 223, the two commits
   of microsoft/openvmm#4984, into position 220. Positions 001-147 keep their positions, v5.1
   positions 149-220 are positions 148-219, and the tree of position 220 is the tree of
-  `3f2ab4c2f`; see [The v6.0 reorganization](#the-v60-reorganization). The pin is 131 commits past
-  the mapped tip on the same first-parent chain. Positions 132-240, 244-246, and 251-262 are not
+  `3f2ab4c2f`; see [The v6.0 reorganization](#the-v60-reorganization). The pin is 132 commits past
+  the mapped tip on the same first-parent chain. Positions 132-240, 244-246, and 251-263 are not
   yet assigned to PRs, positions 241-243 are PR09 fixups, position 247 is a PR13 fixup, position 248
   is a PR18 fixup, position 249 is a PR20 fixup, and position 250 is a PR19 fixup; see
   [Commits after the mapped tip](#commits-after-the-mapped-tip).
@@ -1033,7 +1034,8 @@ ba105dd7f
   -> LAPIC timer restores without drift or a downtime refire [256-258]
   -> LAPIC timer captures that keep a taken timer from refiring [259-261]
   -> restore phase logs through a host clock step back [262]
-  -> c2c21ddb2 (NVX pin)
+  -> PIT low/high counts without a control word [263]
+  -> 5d656d854 (NVX pin)
 ```
 
 For upstream review, PR01-PR06 and PR14 can be rebased independently onto current upstream main,
@@ -1070,15 +1072,15 @@ helper that the case uses.
 
 ## Commits after the mapped tip
 
-The NVX pin extends the chain past position 131 without merges. Of the 131 later commits,
+The NVX pin extends the chain past position 131 without merges. Of the 132 later commits,
 positions 241-243 are PR09 fixups, position 247 is a PR13 fixup, position 248 is a PR18 fixup,
 position 249 is a PR20 fixup, position 250 is a PR19 fixup, and the others are not yet classified
 or assigned to PRs.
 Each of positions 132-186 combines the v2.4 commits that the table lists, as v3.0 positions 145-160
 and 162-200 did, which developed one concept; "(part)" marks a v2.4 fixup that split across
 commits. Positions 187-191 followed the v3.0 reorganization, positions 192-205 are the ABI restart
 of the v4.0 reorganization, positions 206-211 followed it, positions 212-218 followed the v5.0
-reorganization, positions 219 and 220 followed the v5.1 rebase, and positions 221-262 followed the
+reorganization, positions 219 and 220 followed the v5.1 rebase, and positions 221-263 followed the
 v6.0 reorganization, so they combine none, apart from position 220, which combines v5.1 positions
 222 and 223. Each commit has the position that it
 had in v5.1 through position 147, and one less from v5.1 position 149 on; see
@@ -1218,6 +1220,7 @@ at `e1ca353eb`, whose first 210 commits are those of `feature/microvm/release/v4
 | 260 | `9f14b54fd` | - | - | - | - | virt_kvm: keep a restore from firing a taken one-shot LAPIC timer again (microsoft/openvmm#5055) |
 | 261 | `c72fc56a3` | - | - | - | - | virt_kvm: build the LAPIC timer KVM tests for musl (microsoft/openvmm#5055); fold into 260 |
 | 262 | `c2c21ddb2` | - | - | - | - | microVM: saturate the time ABI restore's logged phase durations (microsoft/openvmm#5056) |
+| 263 | `5d656d854` | - | - | - | - | chipset: start each PIT low/high count with its low byte (microsoft/openvmm#5061) |
 
 The remarks that say whether commits apply to upstream main record what the v3.0 reorganization
 found on its base, `b2676222b`, at the positions that the commits have now.
@@ -1884,6 +1887,12 @@ that positions 135, 138, 139, 142, 145, 147-150, 152, 153, 156, 158-160, 193, 20
 main conflicts in all four of its files: upstream main lacks the time ABI, so it follows the time
 ABI series upstream.
 
+Position 263 makes the PIT's count writes in the low-then-high access mode take each count's low
+byte first, in `Timer::write`, code that upstream main has, and adds three tests, in a file that
+PR10's position 022 and position 139 also change, neither of them in the code that it changes. It
+changes nothing that a mapped PR adds. Replaying it on upstream main applies without conflicts,
+and `chipset` type-checks there with its tests, which pass, so it can go upstream on its own.
+
 ## PR summary
 
 Each count is of v6.0 commits, which include the fixups that the v3.0, v4.0, v5.0, and v6.0