Exposing -fw_cfg?

[Martin Kletzander]

On Wed, May 20, 2020 at 01:24:36PM +0100, Daniel P. Berrangé wrote:
>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
>gross.
>
>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.
>

For some other (albeit older) info:

   https://post-office.corp.redhat.com/mailman/private/virt-devel/2017-February/msg00278.html

I also recall someone was thinking this could be usable for cloud-init data.

If someone needs this, then it would be nice to provide some background and/or
justification here:

   https://bugzilla.redhat.com/show_bug.cgi?id=1422831

>Regards,
>Daniel
>-- 
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