From 541fd952e81a3ee0dce6ae79d4e6d0a584d384bb Mon Sep 17 00:00:00 2001
From: Asahi Lina <lina@asahilina.net>
Date: Sun, 27 Oct 2024 01:40:16 +0900
Subject: [PATCH] Share /dev/shm as a separate mount with DAX

Create a new virtiofs filesystem and mount it separately with DAX
unconditionally enabled.

Signed-off-by: Asahi Lina <lina@asahilina.net>
---
 crates/muvm/src/bin/muvm.rs    | 22 ++++++++++++++++++----
 crates/muvm/src/guest/mount.rs | 15 ++++++++++++++-
 2 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/crates/muvm/src/bin/muvm.rs b/crates/muvm/src/bin/muvm.rs
index 5ea4959..1c2d9f7 100644
--- a/crates/muvm/src/bin/muvm.rs
+++ b/crates/muvm/src/bin/muvm.rs
@@ -6,10 +6,10 @@ use std::path::Path;
 
 use anyhow::{anyhow, Context, Result};
 use krun_sys::{
-    krun_add_disk, krun_add_vsock_port, krun_create_ctx, krun_set_env, krun_set_gpu_options2,
-    krun_set_log_level, krun_set_passt_fd, krun_set_root, krun_set_vm_config, krun_set_workdir,
-    krun_start_enter, VIRGLRENDERER_DRM, VIRGLRENDERER_THREAD_SYNC,
-    VIRGLRENDERER_USE_ASYNC_FENCE_CB, VIRGLRENDERER_USE_EGL,
+    krun_add_disk, krun_add_virtiofs2, krun_add_vsock_port, krun_create_ctx, krun_set_env,
+    krun_set_gpu_options2, krun_set_log_level, krun_set_passt_fd, krun_set_root,
+    krun_set_vm_config, krun_set_workdir, krun_start_enter, VIRGLRENDERER_DRM,
+    VIRGLRENDERER_THREAD_SYNC, VIRGLRENDERER_USE_ASYNC_FENCE_CB, VIRGLRENDERER_USE_EGL,
 };
 use log::debug;
 use muvm::cli_options::options;
@@ -231,6 +231,20 @@ fn main() -> Result<()> {
             let err = Errno::from_raw_os_error(-err);
             return Err(err).context("Failed to configure root path");
         }
+
+        // SAFETY: `c_path` and `c_path` are pointers to C-string literals.
+        let err = unsafe {
+            krun_add_virtiofs2(
+                ctx_id,
+                c"devshm".as_ptr(),
+                c"/dev/shm/".as_ptr(),
+                1u64 << 29, // 512MiB should be enough for /dev/shm
+            )
+        };
+        if err < 0 {
+            let err = Errno::from_raw_os_error(-err);
+            return Err(err).context("Failed to configure /dev/shm filesystem");
+        }
     }
 
     {
diff --git a/crates/muvm/src/guest/mount.rs b/crates/muvm/src/guest/mount.rs
index a816b3c..3126ca0 100644
--- a/crates/muvm/src/guest/mount.rs
+++ b/crates/muvm/src/guest/mount.rs
@@ -7,7 +7,8 @@ use std::path::Path;
 use anyhow::{Context, Result};
 use rustix::fs::{mkdir, symlink, Mode, CWD};
 use rustix::mount::{
-    mount2, mount_bind, move_mount, open_tree, MountFlags, MoveMountFlags, OpenTreeFlags,
+    mount2, mount_bind, move_mount, open_tree, unmount, MountFlags, MoveMountFlags, OpenTreeFlags,
+    UnmountFlags,
 };
 
 fn make_tmpfs(dir: &str) -> Result<()> {
@@ -160,5 +161,17 @@ pub fn mount_filesystems() -> Result<()> {
         make_tmpfs("/tmp/.X11-unix")?;
     }
 
+    // Mount /dev/shm separately with DAX enabled, to allow cross-domain shared memory
+    // /dev/shm is mounted by libkrunfw, so unmount it first
+    unmount("/dev/shm", UnmountFlags::empty()).context("Failed to unmount /dev/shm")?;
+    mount2(
+        Some("devshm"),
+        "/dev/shm",
+        Some("virtiofs"),
+        MountFlags::NOEXEC | MountFlags::NOSUID,
+        Some(c"dax"),
+    )
+    .context("Failed to mount `/dev/shm`")?;
+
     Ok(())
 }