Script to run compiled linux kernel with root fs mounted from host

Useful to debug (via either printk or gdb) the kernel - one can boot it,
and tweak compile test program on host and be ready to test it inside
qemu on tested kernel. Another use case is to add tracing printk to
kernel and to boot it on each iteration. Booting via qemu means the
workflow is not disrupted as would with rebooting the hardware.

With this tool it was straightforward to find out why mmap-alias through
mremap does not work for huge pages:

static struct vm_area_struct *vma_to_resize(unsigned long addr,
        unsigned long old_len, unsigned long new_len, unsigned long *p)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma = find_vma(mm, addr);

        if (!vma || vma->vm_start > addr)
                goto Efault;

        if (is_vm_hugetlb_page(vma))            <--
                goto Einval;

mremap_to(...)
{
        /* lots of checks, then */
        vma_to_resize(...)

t/qemu-runlinux

+#!/bin/sh -e
+# qemu-runlinux <kernel>
+# run kernel in QEMU with root fs taken from host
+#
+# Useful to test/debug just compiled kernel via running programs
+# edited/compiled on host.
+
+kernel=$1
+arch=`uname -m`
+test -z "$kernel" && kernel=arch/$arch/boot/bzImage
+
+
+# may be also useful:
+#   -serial  stdio
+#   -serial  file:ttyS0
+#   -monitor stdio
+#   -serial  stdio
+#   -S -gdb tcp::1234
+
+# NOTES
+#   - for kernel to mount root, 9P support must be compiled in:
+#       CONFIG_NET_9P=y
+#       CONFIG_NET_9P_VIRTIO=y
+#       CONFIG_9P_FS=y
+#
+#   - mount_tag *must* be /dev/root - as of 3.17-rc1 the kernel hardcodes it
+#
+# References
+#   http://unix.stackexchange.com/questions/90423/can-virtfs-9p-be-used-as-root-file-system
+#   http://stackoverflow.com/questions/11408041/kernel-debugging-with-gdb-and-qemu
+qemu-system-$arch   \
+    -enable-kvm \
+ \
+    -nographic  \
+ \
+    -fsdev  local,id=R,path=/,security_model=none,readonly  \
+    -device virtio-9p-pci,fsdev=R,mount_tag=/dev/root   \
+ \
+    -kernel $kernel \
+    -append "ro rootfstype=9p rootflags=trans=virtio  console=ttyS0 init=/bin/sh"