libkrun-efi: macos doesn't work with external kernel: `FirmwareInvalidAddress(InvalidGuestAddress(GuestAddress(0)))`

[Kern--]

When using libkrun-efi to start a vm with an extneral kernel, I'm consistently getting this error:

[2025-11-13T22:11:53Z ERROR krun] Building the microVM failed: FirmwareInvalidAddress(InvalidGuestAddress(GuestAddress(0)))

---

Looking through the code, it looks like when using efi, firmware_data and firmware_size are set here:

libkrun/src/vmm/src/builder.rs

Lines 1411 to 1412 in 3847069

	#[cfg(feature = "efi")]
	let (firmware_data, firmware_size) = (Some(EDK2_BINARY), Some(EDK2_BINARY.len()));

But firmware_size is not passed through when initializing the memory regions:

libkrun/src/vmm/src/builder.rs

Lines 1443 to 1447 in 3847069

	#[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]
	let (arch_mem_info, mut arch_mem_regions) = match payload {
	Payload::ExternalKernel(external_kernel) => {
	arch::arch_memory_regions(mem_size, external_kernel.initramfs_size, None)
	}

Then writing the firmware data fails here:

libkrun/src/vmm/src/builder.rs

Lines 1476 to 1480 in 3847069

	if let Some(firmware_data) = firmware_data.as_ref() {
	guest_mem
	.write(firmware_data, GuestAddress(arch_mem_info.firmware_addr))
	.map_err(StartMicrovmError::FirmwareInvalidAddress)?;
	}

I was able to fix it by passing firmware_size through, but I was also able to fix it by setting firmware_data and firmware_size to None. I'm guessing the former is the correct solution, is that right?

diff --git a/src/vmm/src/builder.rs b/src/vmm/src/builder.rs
index c9f5ec6..cc0d00c 100644
--- a/src/vmm/src/builder.rs
+++ b/src/vmm/src/builder.rs
@@ -1443,7 +1443,7 @@ fn create_guest_memory(
     #[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]
     let (arch_mem_info, mut arch_mem_regions) = match payload {
         Payload::ExternalKernel(external_kernel) => {
-            arch::arch_memory_regions(mem_size, external_kernel.initramfs_size, None)
+            arch::arch_memory_regions(mem_size, external_kernel.initramfs_size, firmware_size)
         }
         _ => arch::arch_memory_regions(mem_size, 0, firmware_size),
     };

---

libkrun version: libkrun-efi 1.16.0 (installed via brew slp/krunkit)
macos version: 15.6.1

[nohajc]

I encountered this error too but I thought in the EFI mode, libkrun actually expects an external firmware (to use instead of UEFI). Not an external kernel. AFAIK they’re also loaded at a different offset in guest memory which might affect things.

[Kern--]

Thanks. I got a another confirmation in nerdbox where I found this issue that I should be using either EFI mode or an external kernel, but not both.

Should libkrun return an error when a caller specifies an external kernel in EFI mode, instead?

[slp]

Should libkrun return an error when a caller specifies an external kernel in EFI mode, instead?

Yes, that sounds like a good idea. I also want to get rid of the EFI flavor ASAP.