Add support for command line MMIO devices, at least qemu and

Firecracker pass MMIO virtual devices mapping through the kernel
command line.
This driver is based on Colin Percival's FreeBSD virtio_mmio_cmdline.c
https://github.com/freebsd/freebsd-src/blob/main/sys/dev/virtio/mmio/virtio_mmio_cmdline.c

The following kernel options are needed

options MPBIOS
options MPTABLE_LINUX_BUG_COMPAT

As are these drivers

pv* at pvbus?
virtio* at pv?

Exemple qemu usage on a Linux host to boot a NetBSD guest:

qemu-system-x86_64 \
       -M microvm,x-option-roms=off,rtc=on,acpi=off,pic=off,accel=kvm \
       -m 256 -cpu host -kernel ${KERNEL} \
       -append "root=ld0a console=com rw -v" \
       -device virtio-blk-device,drive=hd0 \
       -drive file=${IMG},format=raw,id=hd0 \
       -device virtio-net-device,netdev=net0 \
       -netdev user,id=net0,ipv6=off,hostfwd=::2200-:22 \
       -global virtio-mmio.force-legacy=false -display none -serial stdio

A lightweight kernel configuration named MICROVM if available for this
use case.

share/man/man4/virtio_mmio.4

@@ -0,0 +1,75 @@
+.\"	$NetBSD: virtio_mmio.4,v 1.1 2025/01/15 13:16:22 imil Exp $
+.\"
+.\" Copyright (c) 2024 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd January 2024
+.Dt VIRTIO_MMIO 4
+.Os NetBSD
+.Sh NAME
+.Nm virtio_mmio
+.Nd VirtIO over memory mapped device.
+.Sh SYNOPSIS
+.Cd "pv* at pvbus?"
+.Cd "virtio* at pv?"
+.Pp
+.Cd "acpi0 at mainbus0"
+.Cd "virtio* at acpi?"
+.Sh DESCRIPTION
+.Nm
+can be used in virtual environments without
+.Xr pci 4
+support
+.Pq a common situation in embedded devices models
+might use simple memory mapped device
+.Pq Nm
+instead of the
+.Xr pci 4
+device.
+.Pp
+The memory mapped
+.Xr virtio 4
+device behaviour is based on the
+.Xr pci 4
+device specification. Therefore most operations including device initialization,
+queues configuration and buffer transfers are nearly identical.
+.Pp
+Unlike
+.Xr pci 4 ,
+.Nm
+provides no generic device discovery mechanism. For each device, the guest OS will
+need to know the location of the registers and interrupt(s) used.
+.Pp
+Device location can be read from either
+.Xr acpi 4
+or via kernel command line parameters, implemented as a
+.Xr pv 4
+virtual device.
+.Sh SEE ALSO
+.Xr virtio 4
+.Pp
+.Rs
+.%T Virtual I/O Device (VIRTIO) Version 1.2
+.%U https://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.html
+.Re

sys/arch/amd64/conf/MICROVM

@@ -0,0 +1,143 @@
+# $NetBSD: MICROVM,v 1.1 2025/01/15 13:16:22 imil Exp $
+#
+# MICROVM kernel configuration, for use with Qemu microvm machine type
+# or Firecracker.
+# Stripped-down configuration with no PCI, use VirtIO over MMIO virtual
+# bus instead. ACPI is disabled as Firecracker doesn't support it,
+# use legacy MP tables instead.
+#
+# Exemple qemu usage on a Linux host to boot a NetBSD guest:
+#
+# qemu-system-x86_64							\
+# 	-M microvm,x-option-roms=off,rtc=on,acpi=off,pic=off,accel=kvm	\
+# 	-m 256 -cpu host -kernel ${KERNEL} 				\
+# 	-append "root=ld0a console=com rw -z"				\
+# 	-display none -device virtio-blk-device,drive=hd0		\
+# 	-drive file=${IMG},format=raw,id=hd0				\
+# 	-device virtio-net-device,netdev=net0 				\
+# 	-netdev user,id=net0,ipv6=off,hostfwd=::2200-:22		\
+# 	-global virtio-mmio.force-legacy=false -serial stdio
+
+machine amd64 x86 xen
+include         "conf/std"      # MI standard options
+include         "arch/xen/conf/std.xenversion"
+
+options         CPU_IN_CKSUM
+options         EXEC_ELF64      # exec ELF binaries
+options         EXEC_SCRIPT     # exec #! scripts
+options         MTRR
+options         MULTIPROCESSOR
+
+options         CHILD_MAX=1024  # 160 is too few
+options         OPEN_MAX=1024   # 128 is too few
+
+mainbus0 at root
+cpu* at mainbus?
+ioapic* at mainbus? apid ?
+
+options 	INCLUDE_CONFIG_FILE	# embed config file in kernel binary
+maxusers	8		# estimated number of users
+
+options 	INSECURE	# disable kernel security levels - X needs this
+
+options 	RTC_OFFSET=0	# hardware clock is this many mins. west of GMT
+
+options 	PIPE_SOCKETPAIR	# smaller, but slower pipe(2)
+
+# Xen PV support for PVH and HVM guests, needed for PVH boot
+options 	XENPVHVM
+options 	XEN
+hypervisor*	at mainbus?		# Xen hypervisor
+xenbus*	 	at hypervisor?		# Xen virtual bus
+xencons*	at hypervisor?		# Xen virtual console
+
+# Include NetBSD 10 compatibility
+options		COMPAT_100
+#
+# Because gcc omits the frame pointer for any -O level, the line below
+# is needed to make backtraces in DDB work.
+#
+makeoptions	COPTS="-O2 -fno-omit-frame-pointer"
+
+# File systems
+#include "conf/filesystems.config"
+file-system FFS
+file-system EXT2FS
+file-system KERNFS
+file-system MFS
+file-system TMPFS
+file-system PTYFS
+file-system MSDOSFS
+file-system PROCFS
+
+options		DKWEDGE_AUTODISCOVER
+options		DKWEDGE_METHOD_GPT
+# File system options
+# ffs
+options 	FFS_NO_SNAPSHOT	# No FFS snapshot support
+options 	WAPBL		# File system journaling support
+
+# Networking options
+#options 	GATEWAY		# packet forwarding
+options 	INET		# IP + ICMP + TCP + UDP
+options 	INET6		# IPV6
+
+# Kernel root file system and dump configuration.
+config		netbsd	root on ? type ?
+
+#
+# Device configuration
+#
+
+# ACPI will be used if present. If not it will fall back to MPBIOS
+options 	MPBIOS			# configure CPUs and APICs using MPBIOS
+# Provide bug-for-bug compatibility with Linux in MP Table searching
+# and parsing.  Firecracker relies on these bugs.
+options		MPTABLE_LINUX_BUG_COMPAT
+
+#pci*	at mainbus? bus ?
+#acpi0	at mainbus0
+
+# ISA bus support
+isa0	at mainbus?
+
+# ISA serial interfaces
+com0	at isa? port 0x3f8 irq 4	# Standard PC serial ports
+
+# Virtual bus for non-PCI devices
+pv* at pvbus?
+
+## Virtio devices
+# Use MMIO by default
+virtio* at pv?
+#virtio* at acpi?
+#virtio* at pci? dev ? function ?	# Virtio PCI device
+#viomb*	at virtio?			# Virtio memory balloon device
+
+ld*	at virtio?			# Virtio disk device
+vioif*	at virtio?			# Virtio network device
+viornd* at virtio?			# Virtio entropy device
+viocon* at virtio?
+
+vio9p*	at virtio?			# Virtio 9P device
+#vioscsi* at virtio?
+#scsibus* at vioscsi?
+
+pseudo-device	md			# memory disk device (ramdisk)
+#options 	MEMORY_DISK_HOOKS	# enable md specific hooks
+#options 	MEMORY_DISK_DYNAMIC	# enable dynamic resizing
+#
+pseudo-device	vnd			# disk-like interface to files
+#options 	VND_COMPRESSION		# compressed vnd(4)
+
+## network pseudo-devices
+pseudo-device	bpfilter		# Berkeley packet filter
+pseudo-device	loop			# network loopback
+
+## miscellaneous pseudo-devices
+pseudo-device	pty			# pseudo-terminals
+# userland interface to drivers, including autoconf and properties retrieval
+pseudo-device	drvctl
+
+file-system	PUFFS
+pseudo-device	putter

sys/arch/x86/pv/files.pv

@@ -2,4 +2,7 @@ define	pvbus {}
 
 device	pv {}
 attach	pv at pvbus
-file	arch/x86/pv/pvbus.c			pvbus	needs-flag
+file	arch/x86/pv/pvbus.c				pvbus	needs-flag
+
+attach	virtio at pv with mmio_cmdline:			virtio_mmio
+file	dev/virtio/arch/x86/virtio_mmio_cmdline.c	mmio_cmdline