vhost-user: encode shmem region IDs as array indices

VhostUserShMemConfig::new() previously accepted a dense slice of sizes,
with region IDs implicitly assigned 0..N. Change it to accept (id, size)
pairs and store each size at memory_sizes[id], leaving absent regions as
zero.

This makes the encoding match crosvm's vmm_vhost, where region IDs are
arbitrary u8 values (e.g. VIRTIO_GPU_SHM_ID_HOST_VISIBLE = 1) rather
than always starting at zero.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

vhost/src/vhost_user/backend_req_handler.rs

@@ -1079,10 +1079,10 @@ mod tests {
 
     #[test]
     fn test_get_shmem_config_multiple_regions() {
-        let memory_sizes = [
-            0x1000, 0x2000, 0x3000, 0x4000, 0x5000, 0x6000, 0x7000, 0x8000,
-        ];
-        let config = VhostUserShMemConfig::new(8, &memory_sizes);
+        let config = VhostUserShMemConfig::new(&[
+            (0, 0x1000), (1, 0x2000), (2, 0x3000), (3, 0x4000),
+            (4, 0x5000), (5, 0x6000), (6, 0x7000), (7, 0x8000),
+        ]);
 
         let (p1, p2) = UnixStream::pair().unwrap();
         let mut dummy_backend = DummyBackendReqHandler::new();
@@ -1109,9 +1109,8 @@ mod tests {
 
     #[test]
     fn test_get_shmem_config_non_continuous_regions() {
-        // Create a configuration with non-continuous regions
-        let memory_sizes = [0x10000, 0, 0x20000, 0, 0, 0, 0, 0];
-        let config = VhostUserShMemConfig::new(2, &memory_sizes);
+        // Create a configuration with non-contiguous region IDs (0 and 2, skipping 1)
+        let config = VhostUserShMemConfig::new(&[(0, 0x10000), (2, 0x20000)]);
 
         let (p1, p2) = UnixStream::pair().unwrap();
         let mut dummy_backend = DummyBackendReqHandler::new();

vhost/src/vhost_user/frontend.rs

@@ -1220,7 +1220,7 @@ mod tests {
     fn test_frontend_get_shmem_config() {
         let (mut frontend, mut peer) = create_pair2();
 
-        let expected_config = VhostUserShMemConfig::new(2, &[0x1000, 0x2000]);
+        let expected_config = VhostUserShMemConfig::new(&[(0, 0x1000), (1, 0x2000)]);
         let hdr = VhostUserMsgHeader::new(
             FrontendReq::GET_SHMEM_CONFIG,
             0x4,

vhost/src/vhost_user/message.rs

@@ -700,7 +700,8 @@ pub struct VhostUserShMemConfig {
     pub nregions: u32,
     /// Padding for correct alignment
     padding: u32,
-    /// Size of each memory region
+    /// Size of each memory region, indexed by region ID.
+    /// A zero entry means the region with that ID is absent.
     pub memory_sizes: [u64; 256],
 }
 
@@ -715,11 +716,17 @@ impl Default for VhostUserShMemConfig {
 }
 
 impl VhostUserShMemConfig {
-    /// Create a new instance
-    pub fn new(nregions: u32, memory: &[u64]) -> Self {
-        let memory_sizes: [u64; 256] = std::array::from_fn(|i| *memory.get(i).unwrap_or(&0));
+    /// Create a new instance from a list of `(id, size)` pairs.
+    ///
+    /// Region IDs do not need to be contiguous. Each ID is used directly as an
+    /// index into the internal array, leaving absent regions as zero.
+    pub fn new(regions: &[(u8, u64)]) -> Self {
+        let mut memory_sizes = [0u64; 256];
+        for &(id, size) in regions {
+            memory_sizes[usize::from(id)] = size;
+        }
         Self {
-            nregions,
+            nregions: regions.len() as u32,
             padding: 0,
             memory_sizes,
         }