microVM: accept host paths with commas in --mount

[ppenna]

This PR lets --mount share a host directory whose path contains commas, for microsoft/nvx#283.

Why

--mount GUEST_TARGET,HOST_PATH[,ro|rw] split its value at its first two commas, so a host path with a comma, such as C:\projects\alpha,beta or C:\reports\2026, final, lost its tail to the mode, which OpenVMM then rejected. Such paths are valid on Windows and Linux, and MXC's filesystem.readonlyPaths and filesystem.readwritePaths accept them. --mount-child already reads its mode after the last comma, so only --mount refused them.

Changes

Area	Change
Command line	--mount reads the guest target up to the first comma and, with another comma, the mode after the last one, so HOST_PATH may contain commas when the mode is given, as with --mount-child. A value with more than one comma must end in ro or rw. The error for another mode suggests ending the value with ,ro or ,rw if HOST_PATH contains commas.
Tests	Command-line tests for host paths with commas, with a trailing comma, and ending in what looks like a mode, and for the error's hint. A resolver test checks that --mount and --mount-child share the exact comma-named directories, alpha,beta and 2026, final, with their modes and denied paths, rather than the sibling alpha that the text before the last comma names.
Guide	The virtio-fs and CLI pages state the rule.

Commit	Content
5a7aad12c	The parser, its tests, and the Guide's virtio-fs and CLI pages.
1feb1f4a3	Review: the hint for an invalid mode is a condition, so it no longer reads as the cause of an error whose host path has no comma, and a test checks it.

Compatibility

Every --mount value that OpenVMM accepted keeps its meaning: with one comma, the host path is shared read-only, and with two, the value still needs a valid mode, so /mnt/share,host,write is still rejected. Only values with three or more commas, which OpenVMM rejected, are newly accepted, and only when they end in ro or rw. --mount-child, snapshots, and saved state are unchanged: snapshots already record canonical host paths as strings.

Validation

The branch sits on main at a5955855c. All results below are for this head, 1feb1f4a3. Its commits are the earlier f62a1126c and 71a528668 rebased onto that main: git range-diff pairs each with its original as identical, and the diffs differ only in the index line of the Guide's virtio-fs page, whose base now includes #132's row.

• On Windows, cargo clippy --all-targets, cargo doc --no-deps, and cargo nextest run --profile agent for openvmm_entry: clean, with 197 tests passed. cargo xtask fmt reports no changes.

• On Linux (prometheus30, bare metal, Azure Linux 3.0), the same nextest run for openvmm_entry: 230 passed.

• filesystem: share host paths that contain commas microsoft/nvx#494, which pins this head, passed on bare-metal Linux/MSHV (prometheus30), Linux/KVM (prometheus32), and Windows/WHP (prometheus28), with OpenVMM built from this head:

  • the filesystem-comma-paths, filesystem-shares, filesystem-policy, denied-filesystem-paths, and filesystem-snapshot scenarios of test-microvm. filesystem-comma-paths shares a read-write alpha,beta and a read-only 2026, final directory through nvx.py run, each alone, as --mount, and both, as --mount-child children. It requires the exact directories, their modes, which hold even after guest root remounts the read-only share read-write, and their hidden denied subdirectories;
  • the aci_edge_sandboxes lifecycle tests, 16 of 16.

  Before the rebase, at 71a528668, it also passed the sandbox share smoke tests with the same two directories, one-shot alone and together, and managed.

The NVX CI covers KVM, MSHV, and WHP. Merge this PR before microsoft/nvx#494.

[Copilot]

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Copilot review overview

2 open findings

• This test case hard-codes a Windows drive-letter path. On non-Windows targets, PathBuf typically… · New
• The added parenthetical applies the 'commas require an explicit mode' guidance to all invalid mode… · New

What changed in this PR

Enables --mount to accept host paths containing commas by parsing the guest target up to the first comma and the mode after the last comma, aligning behavior with --mount-child.

Changes:

• Updated --mount parsing to use first-comma / last-comma splitting and improved error text for invalid modes in comma-containing paths.
• Added unit/integration tests covering comma-containing host paths and edge cases.
• Updated CLI and virtio-fs guide documentation to describe the new parsing rule.

File	Description
openvmm/​openvmm_entry/​src/​microvm/​filesystem.rs	Adds a resolver-style test ensuring mounts and denied paths resolve to the intended comma-named directories.
openvmm/​openvmm_entry/​src/​cli_args/​microvm.rs	Updates --mount parsing to support commas in host paths; adds parsing tests and updates CLI help text.
Guide/​src/​reference/​openvmm/​management/​cli.md	Documents the first-comma / last-comma parsing rule for --mount.
Guide/​src/​reference/​devices/​virtio/​virtio-fs.md	Documents the first-comma / last-comma parsing rule for virtio-fs --mount.

🧠 Review effort: Lite

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