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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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10
vmm/src/vm_config.rs
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@@ -759,6 +759,12 @@ pub enum PayloadConfigError {
/// FwCfg missing initramfs
#[error("Error --fw-cfg-config: missing --initramfs")]
FwCfgMissingInitramfs,
#[cfg(feature = "fw_cfg")]
/// Invalid fw_cfg item content
#[error(
"Error --fw-cfg-config: invalid item '{0}' (exactly one of 'file' or 'string' is required)"
)]
FwCfgInvalidItem(String),
}
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
@@ -819,7 +825,9 @@ pub struct FwCfgItem {
#[serde(default)]
pub name: String,
#[serde(default)]
pub file: PathBuf,
pub file: Option<PathBuf>,
#[serde(default)]
pub string: Option<String>,
}
#[cfg(feature = "fw_cfg")]