From bcc9394d67b33e5ba6ccb236d0e5001f636e4058 Mon Sep 17 00:00:00 2001
From: Manish Ranjan Mahanta <mmahanta@microsoft.com>
Date: Fri, 20 Feb 2026 21:27:02 +0530
Subject: [PATCH] Adding support for ARM64 LCOW

Signed-off-by: Manish Ranjan Mahanta <mmahanta@microsoft.com>
---
 cmd/runhcs/create-scratch.go |  8 ++++++++
 internal/uvm/create_lcow.go  | 28 ++++++++++++++++++++++++----
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/cmd/runhcs/create-scratch.go b/cmd/runhcs/create-scratch.go
index 34a7500a5e..f28cb361b6 100644
--- a/cmd/runhcs/create-scratch.go
+++ b/cmd/runhcs/create-scratch.go
@@ -4,6 +4,7 @@ package main
 
 import (
 	gcontext "context"
+	"runtime"
 
 	"github.com/Microsoft/hcsshim/internal/appargs"
 	"github.com/Microsoft/hcsshim/internal/lcow"
@@ -72,6 +73,13 @@ var createScratchCommand = cli.Command{
 			sizeGB = lcow.DefaultScratchSizeGB
 		}
 
+		if runtime.GOARCH == "arm64" {
+			// Direct-boot is not supported on ARM64, so disable KernelDirect even if the flag is set.
+			opts.KernelDirect = false
+			// ARM64 doesn't support VPMem yet, so disable it even if the flag is set.
+			opts.VPMemDeviceCount = 0
+		}
+
 		convertUVM, err := uvm.CreateLCOW(ctx, opts)
 		if err != nil {
 			return errors.Wrapf(err, "failed to create '%s'", opts.ID)
diff --git a/internal/uvm/create_lcow.go b/internal/uvm/create_lcow.go
index 5ac388b181..de481ef3ea 100644
--- a/internal/uvm/create_lcow.go
+++ b/internal/uvm/create_lcow.go
@@ -10,6 +10,7 @@ import (
 	"net"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 
 	"github.com/Microsoft/go-winio"
@@ -181,6 +182,12 @@ func NewDefaultOptionsLCOW(id, owner string) *OptionsLCOW {
 
 	opts.UpdateBootFilesPath(context.TODO(), defaultLCOWOSBootFilesPath())
 
+	if runtime.GOARCH == "arm64" {
+		// ARM64 doesn't support KernelDirect, so disable it even if the flag is set.
+		opts.KernelDirect = false
+		// ARM64 doesn't support VPMem yet, so disable it even if the flag is set.
+		opts.VPMemDeviceCount = 0
+	}
 	return opts
 }
 
@@ -218,11 +225,12 @@ func (opts *OptionsLCOW) UpdateBootFilesPath(ctx context.Context, path string) {
 		}).Debug("updated LCOW root filesystem to " + VhdFile)
 	}
 
-	if opts.KernelDirect {
+	if opts.KernelDirect && runtime.GOARCH != "arm64" {
 		// KernelDirect supports uncompressed kernel if the kernel is present.
 		// Default to uncompressed if on box. NOTE: If `kernel` is already
 		// uncompressed and simply named 'kernel' it will still be used
 		// uncompressed automatically.
+		// ARM64 doesn't support KernelDirect, so skip this logic in that case.
 		if _, err := os.Stat(filepath.Join(opts.BootFilesPath, UncompressedKernelFile)); err == nil {
 			opts.KernelFile = UncompressedKernelFile
 
@@ -807,14 +815,20 @@ func makeLCOWDoc(ctx context.Context, opts *OptionsLCOW, uvm *UtilityVM) (_ *hcs
 	vmDebugging := false
 	if opts.ConsolePipe != "" {
 		vmDebugging = true
-		kernelArgs += " 8250_core.nr_uarts=1 8250_core.skip_txen_test=1 console=ttyS0,115200"
+		if runtime.GOARCH == "arm64" {
+			kernelArgs += " console=ttyAMA0,115200"
+		} else {
+			kernelArgs += " 8250_core.nr_uarts=1 8250_core.skip_txen_test=1 console=ttyS0,115200"
+		}
 		doc.VirtualMachine.Devices.ComPorts = map[string]hcsschema.ComPort{
 			"0": { // Which is actually COM1
 				NamedPipe: opts.ConsolePipe,
 			},
 		}
 	} else {
-		kernelArgs += " 8250_core.nr_uarts=0"
+		if runtime.GOARCH != "arm64" {
+			kernelArgs += " 8250_core.nr_uarts=0"
+		}
 	}
 
 	if opts.EnableGraphicsConsole {
@@ -835,7 +849,7 @@ func makeLCOWDoc(ctx context.Context, opts *OptionsLCOW, uvm *UtilityVM) (_ *hcs
 		kernelArgs += " " + opts.KernelBootOptions
 	}
 
-	if !opts.VPCIEnabled {
+	if runtime.GOARCH != "arm64" && !opts.VPCIEnabled {
 		kernelArgs += ` pci=off`
 	}
 
@@ -967,6 +981,12 @@ func CreateLCOW(ctx context.Context, opts *OptionsLCOW) (_ *UtilityVM, err error
 		return nil, errors.Wrap(err, errBadUVMOpts.Error())
 	}
 
+	// KernelDirect and VPMem devices are not supported by Hyper-V on ARM64.
+	if runtime.GOARCH == "arm64" {
+		opts.KernelDirect = false
+		opts.VPMemDeviceCount = 0
+	}
+
 	// HCS config for SNP isolated vm is quite different to the usual case
 	var doc *hcsschema.ComputeSystem
 	if opts.SecurityPolicyEnabled {