🔴 Hot News: microsoft/WSL/pull/

Daman Mulye (damanm24) · 2026-07-21T16:16:03Z

With virtio-fs enabled, each drvfs share was backed by its own virtio-fs device. Each device consumes an MMIO aperture, of which there are a fixed amount available for a VM. When a VM runs out of them subsequent calls to mount shares will hang.

To work around this limit, all drvfs shares are backed by a single aggregate virtio-fs device per server (one for the non-admin drvfs, and another for the admin). The aggregate device exposes a synthetic root with one child per share keyed by a GUID subname. Adding a share now adds a child to the existing device instead of creating a new device. In the guest the aggregate device is mounted once at a hidden location (/run/wsl/virtiofs-mounts/drvfsa) and each share is a bind-mount of <device>/<child-name> -> target.

Key Changes:
Wsl Service:

WslCoreVm::AddVirtioFsShare now creates the aggregate device on first use (VirtiofsShareKind_Aggregate) and calls AddVirtiofsChild(device, childName, options, path) for every share
New GuestDeviceManager::AddVirtiofsChild / DeviceHostProxy::AddVirtiofsChild + GetVirtiofsDevice, forwarding to the device-host IWslVirtiofsDevice::AddChild.

Protocol changes:

LX_INIT_ADD_VIRTIOFS_SHARE_RESPONSE_MESSAGE gains ChildNameOffset so the guest learns which child to bind.

Guest init changes:

MountVirtioFs mounts the aggregate root once, then bind-mounts <VIRTIOFS_MOUNT_DIR>// → target. Falls back to Plan 9 if the share-add or the bind fails (so shares on volumes the server can't fully back still mount).
QueryVirtiofsMountSource takes a Root argument to resolve an aggregate child back to its Windows path; config.cpp mount enumeration/remount handles the aggregate (non-/ root) case and skips the internal hidden mounts.
UtilFindMount excludes internal mounts under VIRTIOFS_MOUNT_DIR from reverse (Windows→Linux) translation, so wslpath resolves to the user-facing /mnt/c instead of the internal /run/wsl/virtiofs-mounts/... path.

Copilot

Pull request overview

This PR changes WSL’s virtio-fs-backed drvfs sharing model from “one virtio-fs device per share” to a single aggregate virtio-fs device per server (separate devices for admin vs non-admin), avoiding MMIO aperture exhaustion that can otherwise cause subsequent mounts to hang.

Changes:

Introduces an aggregate virtio-fs device that exposes per-share children keyed by GUID, and updates the host↔guest protocol to return the child name for binding.
Updates Linux init to mount the aggregate root once under a hidden directory and bind-mount per-share children to user-visible targets; updates mount enumeration / reverse path translation to ignore internal mounts.
Updates Windows and Linux tests to reflect aggregate virtio-fs behavior (mount root semantics, mount-id lookup, and exercising read-only mounts).

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
test/windows/DrvFsTests.cpp	Expands virtiofs loop test and adds alternating read-only mounts with validation.
test/linux/unit_tests/lxtmount.c	Adjusts mount root checks and mount-id lookup for aggregate virtio-fs semantics.
test/linux/unit_tests/drvfs.c	Updates test applicability/behavior notes for virtiofs parity with Plan9 limitations.
src/windows/service/exe/WslCoreVm.h	Changes AddVirtioFsShare return type and adds aggregate device instance tracking.
src/windows/service/exe/WslCoreVm.cpp	Implements aggregate virtio-fs device + per-share child creation; includes protocol response updates.
src/windows/common/GuestDeviceManager.h	Adds AddVirtiofsChild API.
src/windows/common/GuestDeviceManager.cpp	Implements AddVirtiofsChild forwarding to the device host.
src/windows/common/DeviceHostProxy.h	Adds AddVirtiofsChild and GetVirtiofsDevice helpers.
src/windows/common/DeviceHostProxy.cpp	Implements virtio-fs device child creation and virtiofs device lookup.
src/shared/inc/lxinitshared.h	Extends add-virtiofs-share response with ChildNameOffset.
src/linux/init/util.cpp	Skips internal aggregate mounts during reverse translation; adjusts virtiofs source resolution.
src/linux/init/drvfs.h	Defines VIRTIOFS_MOUNT_DIR and extends QueryVirtiofsMountSource signature.
src/linux/init/drvfs.cpp	Adds aggregate-root + child bind mounting logic; updates tag mapping and source query behavior.
src/linux/init/config.cpp	Updates mount enumeration/remount logic to handle aggregate virtio-fs and skip internal mounts.
packages.config	Updates Microsoft.WSL.DeviceHost package version.

Brian Perkins (Brian-Perkins) · 2026-07-21T16:42:15Z

I suggest we make this a config option and set it to enabled by default, because (1) there may be issues that require disabling it; (2) longer term we are probably moving back to individual devices so it would probably be good to maintain and test that code. Agreed though that it is a pain to have to test both variations.

Copilot

Pull request overview

Copilot reviewed 19 out of 19 changed files in this pull request and generated 3 comments.

Copilot

Pull request overview

Copilot reviewed 19 out of 19 changed files in this pull request and generated 1 comment.

Ben Hillis (benhillis) · 2026-07-21T21:30:27Z

It does not look like WSLC was updated to use the new logic.

Ben Hillis (benhillis) · 2026-07-21T21:30:56Z

It does not look like WSLC was updated to use the new logic.

That can be done as a follow-up if you like.

Copilot

Pull request overview

Copilot reviewed 19 out of 19 changed files in this pull request and generated no new comments.

Copilot

Pull request overview

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

+        if (UtilIsPathPrefix(MountEnum.Current().MountPoint, VIRTIOFS_MOUNT_DIR, false) > 0)
+        {
+            continue;
+        }


    //
    // Validate the tag is a GUID.
    //



Ben Hillis (benhillis)

Looks good, thanks for doing this!

Now that DrvFs shares are carried by a single aggregate virtiofs device (#41129 / #41151), only a small, fixed number of virtiofs devices exist per VM. That leaves enough memory-aperture headroom to give each aggregate multiple request queues, which significantly improves concurrent DrvFs throughput. Non-aggregate (per-share file-backed) and section-backed devices stay at a single queue: many of them can exist, and raising their queue count would reintroduce the aperture exhaustion that the one-queue default was added to avoid. fio on /mnt/c (16 parallel jobs, psync), 4 queues vs 1: randread 4k +56% IOPS (-36% latency) randwrite 4k +622% IOPS (-85% latency) smallfile randrw +48% IOPS (-33% latency) seqread 1M +15% IOPS seqwrite 1M -10% IOPS (single-stream, bandwidth-bound; within noise) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 4ab64eb9-4271-4241-b838-3da58ec39305

Now that DrvFs shares are carried by a single aggregate virtiofs device (#41129 / #41151), only a small, fixed number of virtiofs devices exist per VM. That leaves enough memory-aperture headroom to give each aggregate multiple request queues, which significantly improves concurrent DrvFs throughput. Non-aggregate (per-share file-backed) and section-backed devices stay at a single queue: many of them can exist, and raising their queue count would reintroduce the aperture exhaustion that the one-queue default was added to avoid. fio on /mnt/c (16 parallel jobs, psync), 4 queues vs 1: randread 4k +56% IOPS (-36% latency) randwrite 4k +622% IOPS (-85% latency) smallfile randrw +48% IOPS (-33% latency) seqread 1M +15% IOPS seqwrite 1M -10% IOPS (single-stream, bandwidth-bound; within noise) Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 4ab64eb9-4271-4241-b838-3da58ec39305

Brian Perkins and others added 10 commits July 13, 2026 17:33

Use bespoke interfaces to communicate with wsldevicehost

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Merge branch 'master' into user/damanmulye/aggregate_share

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revert

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touch up

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test fixes

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restore

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Daman Mulye added 2 commits July 21, 2026 12:03

address review comments

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Use feature flag to gate aggregate share changes

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Comment thread src/linux/init/util.cpp

Comment on lines +889 to +892

if (UtilIsPathPrefix(MountEnum.Current().MountPoint, VIRTIOFS_MOUNT_DIR, false) > 0)

{

continue;

}

Comment thread src/linux/init/drvfs.cpp

Comment on lines 905 to 908

//

// Validate the tag is a GUID.

//

Ben Hillis (benhillis) approved these changes Jul 22, 2026

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Daman Mulye (damanm24) merged commit 9e58620 into master Jul 22, 2026
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Daman Mulye (damanm24) deleted the user/damanmulye/aggregate_share branch July 22, 2026 15:06

Daman Mulye (damanm24) mentioned this pull request Jul 23, 2026

virtiofs: Use aggregate share feature in WSLC #41151

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Ben Hillis (benhillis) mentioned this pull request Jul 24, 2026

Use 4 request queues for aggregate virtiofs devices #41169

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