Exposing -fw_cfg?

Daniel P. Berrangé berrange at redhat.com
Wed May 20 12:24:36 UTC 2020

On Wed, May 20, 2020 at 02:16:24PM +0200, Michal Privoznik wrote:
> List,
> QEMU has capability to inject various blobs into firmware that configure how
> firmware configures itself. However, it can be also used to passthrough a
> specific file into the guest. For instance:
>   -fw_cfg value=name=opt/com.example,file=/tmp/ign
> will make the /tmp/ign file accessible in the guest under:
>   /sys/firmware/qemu_fw_cfg/by_name/opt/com.example/raw
> the @name is important here as it defines what knob is touched. For
> instance, /opt/ovmf tweaks OVMF, /bootorder changes the boot order, and so
> on. But, if the @name is in /opt/reverse.fully.qualified.domain form than
> this is a blob that is exposed into the guest and does not affect ACPI,
> SMBIOS, ... And IMO this is what makes the interface horrible.
> While I definitely would not expose the FW configuration knobs (we already
> provide a way to configure things like bootroder), the file passthrough is
> actually used. So far I have found out that RHCOS uses it to give the guest
> so called ignition file (for the sake of argument we can assume it's like a
> kickstart that the OS reads on the first boot and configures itself up), but
> there are some other potential users (for users it looks intriguing, it's a
> simple API that makes a file show up at well defined location inside the
> guest).

FYI, the QEMU maintainers do not want to see applications using fw-cfg
for general purpose data passthrough.  This interface is primarily there
for QEMU to communicate with the BIOS. There are a limited number of data
slots available, so they're reasonably precious, and QEMU is using more
over time.

This is the key reason why I implemented support for "OEM Strings" feature
in the QEMU. From POV of an application, this provides the same functionality
as fw_cfg, but via SMBIOS data tables instead, so doesn't have the  same
limits as fw_cfg.  This data can be queried using the "dmidecode" tool in
the guest eg   "dmidecode --oem-string count", and
"dmidecode --oem-string NNN". You could just parse the raw SMBOIS table
from sysfs instead and ignore "dmidecode" tool, but that's a bit more

Linux exposes some, but not all, SMBIOS fields in sysfs, but lacks OEM
strings currently. I made a half-hearted attempt to add linux support
for OEM strings in sysfs, but never completed it.

> Therefore I vouch for exposing the file passthorugh (and definitely do not
> mention firmware in the element or docs in any way, to not encourage users
> to use FW tweaking mode). However, before I design something, I'd like to
> hear your opinion.

I don't object to exposing fw_cfg in the XML, since there are existing
users like Ignition that could benefit.  I think we should be documenting
that its usage is strongly discouraged though, in favour of OEM strings.

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