On Wed, May 20, 2020 at 02:39:25PM +0200, Martin Kletzander wrote: > 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 I think this page shows the motivation: https://coreos.com/os/docs/latest/booting-with-libvirt.html They're telling people to use QEMU command line passthrough, which will also need special handling for sVirt snce libvirt won't know about it. <tangent> .... BTW, it occurred to me that we should improve the QEMU passthrough to deal with sVirt, eg <qemu:commandline> <qemu:arg value="-fw_cfg"/> <qemu:arg value="name=opt/com.coreos/config,file=/var/lib/libvirt/container-linux/container-linux1/provision.ign"/> <qemu:access path="/var/var/libvirt/container-lonux/container-linux1/provision.ign"/> </qemu:commandline> <qemu:permit/> would add the default read-write sVirt label but could access an <seclabel> child element to override. We could also have APIs to make monitor passthrough more viable virDomainQemuPermitAccess(virDOmainPtr dom, const char *path, unsigned int flags) virDomainQemuRevokeAccess(virDOmainPtr dom, const char *path, unsigned int flags) </tangent> 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 :|
