docs: update QEMU mode docs for =PATH syntax and uncompressed initramfs

README.md

@@ -38,7 +38,7 @@ ecr [OPTIONS] <DISTRO[:VERSION]> [-- COMMAND...]
 | `--no-cache` | Force a fresh download, bypassing the cache |
 | `-v, --verbose` | Print diagnostic output (URLs, layer info, extraction steps) |
 | `-a, --arch <ARCH>` | Target architecture (`amd64`, `arm64`, `armhf`, `riscv64`, …) |
-| `--kernel [PATH]` | Boot with QEMU system emulation. Downloads Alpine's `linux-virt` kernel if no path provided |
+| `--kernel[=PATH]` | Boot with QEMU system emulation. Downloads Alpine's `linux-virt` kernel if no `=PATH` given |
 | `-m, --memory <SIZE>` | Memory for QEMU VM (default: 2G, only with `--kernel`) |
 
 ## Examples
@@ -62,8 +62,8 @@ ecr --no-cache fedora
 # Boot with QEMU system emulation (auto-downloads default kernel)
 ecr --kernel alpine
 
-# Boot with your own kernel
-ecr --kernel /boot/vmlinuz ubuntu
+# Boot with your own kernel (note the `=` — a space would parse the path as the distro)
+ecr --kernel=/boot/vmlinuz ubuntu
 
 # Boot with custom memory
 ecr --kernel --memory 4G alpine
@@ -78,16 +78,18 @@ When `--kernel` is specified, ecr boots the rootfs in a full QEMU virtual machin
 ecr --kernel alpine
 
 # Use your own kernel
-ecr --kernel /boot/vmlinuz ubuntu
+ecr --kernel=/boot/vmlinuz ubuntu
 ```
 
 This mode:
-- Creates a gzipped CPIO initramfs from the rootfs
+- Creates an uncompressed CPIO initramfs from the rootfs (streamed to disk)
 - Boots QEMU with your kernel (or auto-downloads Alpine's `linux-virt` kernel)
 - Provides full VM isolation
 - Works for any architecture (no binfmt_misc needed)
 - Caches the default kernel in `~/.cache/ecr/`
 
+Host bind mounts (`--bind`, `--bind-rw`) are not applied in this mode.
+
 Requirements:
 - `qemu-system-<arch>` installed
 - For custom kernels: kernel must have serial console support

SPEC.md

@@ -22,7 +22,7 @@ ecr [OPTIONS] <DISTRO[:VERSION]> -- [COMMAND]...
 | `--bind-rw <path>` | none | Read-write bind mount at `/mnt/<basename>` (can be specified multiple times, overrides `--bind` for same path) |
 | `--no-cache` | false | Download fresh tarball, ignore cache |
 | `--no-bind` | false | Skip mounting any directory |
-| `--kernel <path>` | none | Boot with QEMU system emulation using specified kernel (triggers disk image creation) |
+| `--kernel[=PATH]` | none | Boot with QEMU system emulation; downloads the default Alpine `linux-virt` kernel when no `=PATH` is given |
 | `-m, --memory <size>` | 2G | Memory size for QEMU VM (only used with `--kernel`) |
 | `-v, --verbose` | false | Print diagnostic messages |
 | `-h, --help` | - | Show help |
@@ -187,35 +187,38 @@ No action required. Modern qemu-user-static packages register binfmt_misc with t
 
 ## QEMU System Emulation Mode
 
-When `--kernel` is specified, ecr switches from namespace/chroot mode to QEMU system emulation. The extracted rootfs is converted to a gzipped CPIO initramfs and booted with the provided kernel.
+When `--kernel` is specified, ecr switches from namespace/chroot mode to QEMU system emulation. The extracted rootfs is converted to an uncompressed CPIO initramfs and booted with the provided kernel.
 
 ### Usage
 
+The kernel path uses `=` syntax (`--kernel=PATH`); `--kernel` without a value downloads the default kernel. Without `=`, a following path would be parsed as the DISTRO argument.
+
 ```sh
-ecr --kernel /boot/vmlinuz ubuntu:noble
-ecr --kernel /boot/vmlinuz --memory 4G alpine
-ecr --kernel /boot/vmlinuz debian -- /bin/sh -c "echo hello"
+ecr --kernel ubuntu:noble
+ecr --kernel=/boot/vmlinuz ubuntu:noble
+ecr --kernel=/boot/vmlinuz --memory 4G alpine
+ecr --kernel=/boot/vmlinuz debian -- /bin/sh -c "echo hello"
 ```
 
 ### Execution Flow
 
 1. Download/cache rootfs tarball (same as namespace mode)
 2. Extract tarball to temporary directory
-3. Create gzipped CPIO initramfs from rootfs
+3. Create uncompressed CPIO initramfs from rootfs (streamed to disk), including essential device nodes (/dev/ttyS0, /dev/null, /dev/tty) and an `/init` script that mounts proc/sys/dev, sets the hostname, execs the requested command argv verbatim (each argv element base64-encoded in the cmdline as `ECR_ARGV`), and powers off on exit
 4. Launch QEMU with:
-   - `-kernel <path>` - provided kernel
-   - `-initrd initramfs.cpio.gz` - rootfs as initramfs
-   - `-append "console=ttyS0 quiet rdinit=/bin/sh -- -c \"setsid sh -c 'exec sh </dev/ttyS0 >/dev/ttyS0 2>&1'\""` - kernel command line
-7. Essential device nodes (/dev/ttyS0, /dev/null, /dev/tty) are added to initramfs for proper console support
+   - `-kernel <path>` - provided (or downloaded) kernel
+   - `-initrd initramfs.cpio` - rootfs as initramfs
+   - `-append "console=ttyS0 [quiet] ECR_SHELL=... [ECR_ARGV=...] ECR_HOSTNAME=..."` - kernel command line (`quiet` unless `-v`)
    - `-m <memory>` - memory size (default 2G)
    - `-display none -serial mon:stdio` - console on stdio
-   - `-netdev user,id=net0 -device virtio-net-pci,netdev=net0` - network
-5. Wait for QEMU to exit
+   - `-netdev user,id=net0 -device virtio-net-pci,netdev=net0` - network NIC
+   - `-enable-kvm -cpu host` - when the host supports KVM and the target matches the host architecture
+5. Wait for QEMU to exit (init powers the VM off when the command/shell exits; `-no-reboot` makes QEMU terminate)
 6. Cleanup temporary files
 
 ### Initramfs Creation
 
-The rootfs directory is converted to a gzipped CPIO archive (newc format) using  the `cpio` crate.
+The rootfs directory is converted to an uncompressed CPIO archive (newc format) using the `cpio` crate, streamed entry by entry so large rootfs images never need to fit in memory. Hard links are preserved: the first occurrence of a (device, inode) pair carries the data with a synthetic inode, subsequent occurrences are zero-size entries sharing that inode, which the kernel's initramfs loader turns into real hard links.
 
 ### Architecture Support
 
@@ -238,11 +241,11 @@ The rootfs directory is converted to a gzipped CPIO archive (newc format) using
 | Feature | Namespace Mode | QEMU Mode |
 |---------|---------------|-----------|
 | Isolation | User namespace | Full VM |
-| Performance | Near-native | Emulated (slower) |
-| Root access | No | No |
+| Performance | Near-native | Emulated (KVM-accelerated when available) |
+| Root access | No | Yes (inside the VM) |
 | Foreign arch | binfmt_misc required | Built-in emulation |
-| Bind mounts | Overlay/bind | Not supported |
-| Network | Host network | User-mode network |
+| Bind mounts | Overlay/bind | Not supported (flags are ignored with a warning) |
+| Network | Host network | User-mode NIC (not configured inside the guest) |
 
 ## File Handling