[Lencerf]

A virtio worker holds RamBus::lock_layout() for as long as its device runs,
while any change of the guest RAM layout takes the write lock. A change while a
device runs therefore blocks until that device resets. For example, a vCPU
hangs when the guest enables the DAX window BAR of a virtio-fs device while a
virtio-blk device serves the root filesystem, and so does hot-plugging a device
with such a BAR.

Turn RamBus into an RCU cell of Ram snapshots:

• A reader calls RamBus::load() to get an Arc<Ram> snapshot, and holds the
  read lock only to clone the Arc.
• A writer calls RamBus::update(), which copies the layout, changes the copy
  and publishes it. A failed update publishes nothing, and Memory changes all
  RAM ranges of a region in one update.

A snapshot holds the ArcMemPages it maps, so the host mappings stay valid as
long as anyone uses the snapshot, even after they are removed from the bus.

A running worker keeps using the snapshot taken when its device was activated,
so it does not see later layout changes yet. Follow-up changes let virtqueues
own their snapshot and move to new layouts between events, wake idle workers
after an update, and let the vhost-user backend accept memory changes while the
device runs.

The commit passes cargo +nightly fmt --check, cargo clippy --all-targets -D warnings on x86_64-unknown-linux-gnu/musl, aarch64-unknown-linux-gnu and
aarch64-apple-darwin, and cargo test --workspace locally.

[coveralls]

Coverage Report for CI Build 42779402185

Coverage increased (+0.003%) to 41.758%

Details

• Coverage increased (+0.003%) from the base build.
• Patch coverage: 17 uncovered changes across 3 files (41 of 58 lines covered, 70.69%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
alioth/src/mem/mem.rs	34	20	58.82%
alioth/src/virtio/vu/backend.rs	2	0	0.0%
alioth/src/cpu/cpu_amd64/sev.rs	1	0	0.0%
Total (5 files)	58	41	70.69%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	16429
Covered Lines:	6644
Line Coverage:	40.44%
Relevant Branches:	1740
Covered Branches:	943
Branch Coverage:	54.2%
Branches in Coverage %:	Yes
Coverage Strength:	901.05 hits per line

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💛 - Coveralls