fix(aarch64/fdt): add dma-coherent property to virtio-mmio nodes

Add the "dma-coherent" property to virtio-mmio nodes in the aarch64 FDT.
This ensures that the guest allocates virtio queues with a Write-Back
memory attribute on platforms that do not support Stage-2 Forced
Write-Back (FWB), maintaining cache coherency between the guest driver
and Firecracker's accesses to the virtio queue.

Signed-off-by: mzh <[email protected]>

Author: Marshal1l

CHANGELOG.md

@@ -56,6 +56,9 @@ and this project adheres to
 - [#5494](https://github.com/firecracker-microvm/firecracker/pull/5494): Fixed a
   watchdog soft lockup bug on microVMs restored from snapshots by calling
   KVM_KVMCLOCK_CTRL ioctl before resuming.
+- [#5538](https://github.com/firecracker-microvm/firecracker/pull/5538): Fixed a
+  cache coherency issue on non-FWB aarch64 platforms by adding `dma-coherent`
+  property to virtio-mmio nodes in the FDT.
 
 ## [1.13.0]
 

src/vmm/src/arch/aarch64/fdt.rs

@@ -377,6 +377,10 @@ fn create_psci_node(fdt: &mut FdtWriter) -> Result<(), FdtError> {
 fn create_virtio_node(fdt: &mut FdtWriter, dev_info: &MMIODeviceInfo) -> Result<(), FdtError> {
     let virtio_mmio = fdt.begin_node(&format!("virtio_mmio@{:x}", dev_info.addr))?;
 
+    // Adding the dma-coherent property ensures that the guest driver allocates the virtio
+    // queue with the Write-Back attribute, maintaining cache coherency with Firecracker's
+    // accesses to the virtio queue.
+    fdt.property_null("dma-coherent")?;
     fdt.property_string("compatible", "virtio,mmio")?;
     fdt.property_array_u64("reg", &[dev_info.addr, dev_info.len])?;
     fdt.property_array_u32(