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vmm, devices: Add fw_cfg string item support

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QEMU supports passing inline string values to the guest via fw_cfg
(-fw_cfg name=...,string=...). Cloud Hypervisor previously only
supported file-backed fw_cfg items. This adds the 'string' option
so users can pass values like OVMF's X-PciMmio64Mb without creating
a temporary file on the host.

Each fw_cfg item now accepts exactly one of 'file' or 'string'.
The FwCfgInvalidItem invariant is validated in PayloadConfig::validate()
(via FwCfgConfig::validate()), covering both CLI and JSON API paths.
The populate_fw_cfg match arm uses unreachable!() since validation
guarantees the invariant holds at that point.

CLI syntax:
  --fw-cfg-config items=[name=opt/ovmf/X-PciMmio64Mb,string=262144]

Signed-off-by: Keith Adler <kadler@cloudflare.com>
This commit is contained in:
Keith Adler
2026-04-14 14:48:49 -05:00
committed by Rob Bradford
parent e4e3375a8d
commit 926dd1e141
6 changed files with 209 additions and 24 deletions
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19
docs/fw_cfg.md
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@@ -39,9 +39,10 @@ The `fw_cfg` device is configured using the `--fw-cfg-config` command-line optio
* `cmdline=on|off`: (Default: `on`) Whether to add the kernel command line (specified by `--cmdline`) to `fw_cfg`.
* `initramfs=on|off`: (Default: `on`) Whether to add the initramfs image (specified by `--initramfs`) to `fw_cfg`.
* `acpi_table=on|off`: (Default: `on`) Whether to add generated ACPI tables to `fw_cfg`.
* `items=[... : ...]`: A list of custom key-value pairs to be exposed via `fw_cfg`.
* `items=[... : ...]`: A list of custom key-value pairs to be exposed via `fw_cfg`. Multiple items are separated by `:`.
* `name=<guest_sysfs_path>`: The path under which the item will appear in the guest's sysfs (e.g., `opt/org.example/my-data`).
* `file=<host_file_path>`: The path to the file on the host whose content will be provided to the guest for this item.
* `file=<host_file_path>`: The path to a file on the host whose content will be provided to the guest for this item.
* `string=<value>`: An inline string value to provide to the guest for this item. Each item must have exactly one of `file` or `string`, not both.
**Example Usage:**
@@ -57,7 +58,19 @@ The `fw_cfg` device is configured using the `--fw-cfg-config` command-line optio
```
In the guest, `/tmp/guest_setup.txt` from the host will be accessible at `/sys/firmware/qemu_fw_cfg/by_name/opt/org.mycorp/setup_info/raw`.
2. **Disabling `fw_cfg` explicitly:**
2. **Inline string items (e.g., OVMF MMIO64 configuration for GPU passthrough):**
```bash
cloud-hypervisor \
--firmware /path/to/OVMF.fd \
--disk path=/path/to/rootfs.img \
--device path=/sys/bus/pci/devices/0000:41:00.0 \
--fw-cfg-config items=[name=opt/ovmf/X-PciMmio64Mb,string=262144] \
...
```
The string `262144` is passed directly to the guest as the content of `opt/ovmf/X-PciMmio64Mb`.
3. **Disabling `fw_cfg` explicitly:**
```bash
cloud-hypervisor \