wslservice: Add experimental.swiotlb .wslconfig setting by asherkariv · Pull Request #40518 · microsoft/WSL

asherkariv · 2026-05-13T01:18:24Z

Add a new experimental .wslconfig key "experimental.swiotlb" that configures SWIOTLB bounce buffer parameters for WSL VMs using virtio devices. The setting is validated, propagated to wsldevicehost.dll via control tokens in the share name, and (for range mode) also passed to the guest kernel command line.

Supported values:

"0x,K|M" (range mode): explicit bounce buffer location Example: experimental.swiotlb=0x100000000,64M

Behavior:

Validated against regex at config parse time; invalid values emit a user warning and are ignored
Requires virtiofs to be enabled; emits a warning and clears the setting if virtiofs is disabled
Appends "hv_pci_swiotlb=," to the kernel command line so the guest driver can locate the bounce buffer
Append ";swiotlb=" to the virtiofs mount options string, which wsldevicehost.dll parses as a control configuration token and strips before passing mount options to the guest

Files changed:

WslCoreConfig.h/cpp: SwiotlbCfg fields, parsing, validation, virtiofs dependency check
WslCoreVm.cpp: kernel cmdline and virtiofs share name plumbing
Resources.resw: error/warning message strings

Related work items: https://microsoft.visualstudio.com/OS/_workitems/edit/61002501

Summary of the Pull Request

PR Checklist

Closes: Link to issue #xxx
Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
Tests: Added/updated if needed and all pass
Localization: All end user facing strings can be localized
Dev docs: Added/updated if needed
Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Add a new experimental .wslconfig key "experimental.swiotlb" that configures SWIOTLB bounce buffer parameters for WSL VMs using virtio devices. The setting is validated, propagated to wsldevicehost.dll via control tokens in the share name, and (for range mode) also passed to the guest kernel command line. Supported values: - "0x<gpa>,<size>m|k" (range mode): explicit bounce buffer location Example: experimental.swiotlb=0x100000000,64M - "lowmem" (lowmem mode): heuristic sub-4GB allocation Behavior: - Validated against regex at config parse time; invalid values emit a user warning and are ignored - Requires virtiofs to be enabled; emits a warning and clears the setting if virtiofs is disabled - Range mode appends "hv_pci_swiotlb=<gpa>,<size>" to the kernel command line so the guest driver can locate the bounce buffer - Both modes append ";swiotlb=<value>" to the virtiofs mount options string, which wsldevicehost.dll parses as a control configuration token and strips before passing mount options to the guest Files changed: - WslCoreConfig.h/cpp: SwiotlbCfg/SwiotlbKernelCfg fields, parsing, validation, virtiofs dependency check - WslCoreVm.cpp: kernel cmdline and virtiofs share name plumbing - wslservice.idl: LXSS_VM_KERNEL_SWIOTLB_CONFIG constant - lxinitshared.h: LX_INIT_SWIOTLB_MOUNT_OPTIONS constant - Resources.resw: error/warning message strings

Copilot

Pull request overview

This PR introduces a new experimental .wslconfig key (experimental.swiotlb) to control SWIOTLB bounce buffer behavior for WSL VMs using virtio devices. The setting is validated during config parsing, optionally propagated to the guest kernel command line (range mode), and also forwarded to wsldevicehost.dll via a virtiofs “name-with-options” control token.

Changes:

Add experimental.swiotlb parsing/validation and store both a virtiofs token value (SwiotlbCfg) and an optional kernel cmdline value (SwiotlbKernelCfg).
Plumb SWIOTLB configuration into virtiofs share options and (range mode) append hv_pci_swiotlb= to the guest kernel command line.
Add new constants and localization strings for invalid-value and virtiofs-required warnings.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/windows/service/inc/wslservice.idl	Adds `LXSS_VM_KERNEL_SWIOTLB_CONFIG` constant used when building the kernel cmdline.
src/windows/service/exe/WslCoreVm.cpp	Appends SWIOTLB kernel parameter (range mode) and adds virtiofs control token in share options.
src/windows/common/WslCoreConfig.h	Adds config key name and new config fields (`SwiotlbCfg`, `SwiotlbKernelCfg`) + telemetry presence bit.
src/windows/common/WslCoreConfig.cpp	Implements `experimental.swiotlb` parsing/validation and enforces virtiofs dependency.
src/shared/inc/lxinitshared.h	Adds `LX_INIT_SWIOTLB_MOUNT_OPTIONS` mount-option/control-token prefix.
localization/strings/en-US/Resources.resw	Adds localized warning strings for invalid SWIOTLB and virtiofs-required cases.

PR comments: experimental.swiotlb plumbing change. Behavior: * Default SwiotlbCfg to "lowmem" so virtiofs gets a SWIOTLB region even when the user does not set experimental.swiotlb explicitly. * Clear SwiotlbCfg/SwiotlbKernelCfg at the top of parseSwiotlb so a malformed value does not silently fall back to the previous (now non-empty) default; instead the feature is disabled and the "Invalid SWIOTLB value ..." warning is emitted. Cleanup: * Inline the ";swiotlb=" mount-option token at its single use site in WslCoreVm::AddDrvFsShare and drop the LX_INIT_SWIOTLB_MOUNT_OPTIONS macro from src/shared/inc/lxinitshared.h (the macro was only referenced once and is not needed by Linux-side code). Tests: * Add experimental.swiotlb cases to WSL2_TEST_METHOD(Warnings) in test/windows/UnitTests.cpp, covering: - valid gpa,size value with virtiofs disabled (default) -> "requires virtiofs" warning - valid "lowmem" with virtiofs disabled -> "requires virtiofs" warning - malformed value ("garbage") and unsupported size suffix ("0x100000000,64g") -> "Invalid SWIOTLB value ..." warning - valid gpa,size value and "lowmem" with virtiofs enabled (DrvFsMode::VirtioFs prefix) -> no warnings

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

…(virtio-net) path.

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

benhillis · 2026-05-13T23:57:12Z

I might be confused but I thought we didn’t need this setting until we have the updated kernel?

- Tighten regex to gpa,size only and add c_swiotlbDefault constant. - Remove SwiotlbKernelCfg field, LXSS_VM_KERNEL_SWIOTLB_CONFIG, and the separate kernel-pattern match; SwiotlbCfg now drives both consumers. - Only emit swiotlb=force / hv_pci_swiotlb= when SwiotlbCfg is set. - Update warning string and tests to match.

A user-supplied kernel may not accept hv_pci_swiotlb=, so clear SwiotlbCfg and emit MessageSwiotlbIncompatibleWithCustomKernel when wsl2.kernel is non-empty. Refactor Initialize to use the EMIT_USER_WARNING + VALIDATE_CONFIG_OPTION pattern (matching EnableVirtio9p) for both the kernel and virtio-consumer prerequisites; the default region is then applied only on the positive path.lease enter the commit message for your changes. Lines starting

Custom kernels are built on the WSL kernel base, which carries the required swiotlb patch, so the gate is unnecessary. Removes the KernelPath check and the MessageSwiotlbIncompatibleWithCustomKernel resource string. The EMIT_USER_WARNING + VALIDATE_CONFIG_OPTION style introduced alongside the gate is kept.

Copilot AI review requested due to automatic review settings May 13, 2026 01:18

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asherkariv added 2 commits May 13, 2026 13:13

Address Clang-format errors.

a450dda

Copilot AI review requested due to automatic review settings May 13, 2026 21:49

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asherkariv added 2 commits May 13, 2026 16:07

Apply experimental.swiotlb default at runtime, and cover VirtioProxy …

1777539

…(virtio-net) path.

Fix clang format errors.

a2dfb8e

benhillis requested a review from Copilot May 13, 2026 23:52

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Comment thread test/windows/UnitTests.cpp Outdated

asherkariv added 3 commits May 14, 2026 13:54

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wslservice: Add experimental.swiotlb .wslconfig setting#40518

wslservice: Add experimental.swiotlb .wslconfig setting#40518
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asherkariv commented May 13, 2026 •

edited

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benhillis commented May 13, 2026

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asherkariv commented May 13, 2026 •

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