fix(vmcall_raw): chunk send to honor VMCALL buffer MTU

src/devices/vmcall_raw/src/stream.rs

@@ -4,7 +4,7 @@
 
 use crate::transport::vmcall::{
     vmcall_raw_transport_can_recv, vmcall_raw_transport_dequeue, vmcall_raw_transport_enqueue,
-    vmcall_raw_transport_init, VMCALL_MIG_CONTEXT_FLAGS,
+    vmcall_raw_transport_init, VMCALL_MIG_CONTEXT_FLAGS, VMCALL_RAW_SEND_PAYLOAD_MTU,
 };
 use core::sync::atomic::AtomicBool;
 
@@ -61,7 +61,17 @@ impl VmcallRaw {
     }
 
     pub async fn send(&mut self, buf: &[u8], _flags: u32) -> Result<usize> {
-        let _ = vmcall_raw_transport_enqueue(self, buf).await?;
+        // A single Service.MigTD.Send VMCALL can only carry one MTU-sized
+        // payload (the same cap the receive side advertises). Chunk here so
+        // callers that don't loop on partial writes (TLS, SPDM transport)
+        // keep working when the pre-session policy + issuer chain blob
+        // exceeds one VMCALL.
+        let mut sent = 0;
+        while sent < buf.len() {
+            let end = core::cmp::min(buf.len(), sent + VMCALL_RAW_SEND_PAYLOAD_MTU);
+            let _ = vmcall_raw_transport_enqueue(self, &buf[sent..end]).await?;
+            sent = end;
+        }
         Ok(buf.len())
     }
 

src/devices/vmcall_raw/src/transport/vmcall.rs

@@ -18,7 +18,16 @@ use td_payload::arch::idt::InterruptStack;
 use td_payload::mm::shared::SharedMemory;
 use tdx_tdcall::tdx;
 
-const MAX_VMCALL_RAW_STREAM_MTU: usize = 0x1000 * 16;
+pub(crate) const MAX_VMCALL_RAW_STREAM_MTU: usize = 0x1000 * 16;
+/// GHCI 1.5 buffer header overhead (8-byte status + 4-byte length) prepended
+/// by `vmcall_raw_transport_enqueue` to each VMCALL payload.
+pub(crate) const VMCALL_RAW_GHCI_HEADER_LEN: usize = 12;
+/// Largest payload that can ride in a single `Service.MigTD.Send` VMCALL.
+/// The VMM-side buffer for the migration data channel matches the MTU the
+/// receive side advertises; sending more than this in one VMCALL fails with
+/// TDX_VMCALL_STATUS != SUCCESS.
+pub(crate) const VMCALL_RAW_SEND_PAYLOAD_MTU: usize =
+    MAX_VMCALL_RAW_STREAM_MTU - VMCALL_RAW_GHCI_HEADER_LEN;
 const VMCALL_VECTOR: u8 = 0x52;
 
 lazy_static! {