Re: Exposing -fw_cfg?

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-Febru...

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 >-- >|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| >|: https://libvirt.org -o- https://fstop138.berrange.com :| >|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| >