Re: [PATCH] docs: Discourage users from using fwcfg

On Tue, Sep 08, 2020 at 11:02:10AM +0200, Laszlo Ersek wrote: > On 09/07/20 15:48, Michal Privoznik wrote: >> Even though this was brought up in upstream discussion [1] it >> missed my patches: users should prefer <oemStrings/> over fwcfg. >> The reason is that fwcfg is considered somewhat internal to QEMU >> and it has limited number of slots and neither of these applies >> to <oemStrings/>. >> >> While I'm at it, I'm fixing the example too (because it contains >> incorrect element name) and clarifying sysfs/ exposure. >> >> 1: https://www.redhat.com/archives/libvir-list/2020-May/msg00957.html >> >> Reported-by: Laszlo Ersek <lersek(a)redhat.com&gt; >> Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; >> --- >> docs/formatdomain.rst | 14 +++++++++----- >> 1 file changed, 9 insertions(+), 5 deletions(-) >> >> diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst >> index 1979dfb8d3..821ffe8d60 100644 >> --- a/docs/formatdomain.rst >> +++ b/docs/formatdomain.rst >> @@ -509,18 +509,22 @@ layout of sub-elements, with supported values of: >> Some hypervisors provide unified way to tweak how firmware configures itself, >> or may contain tables to be installed for the guest OS, for instance boot >> order, ACPI, SMBIOS, etc. It even allows users to define their own config >> - blobs. In case of QEMU, these then appear under domain's sysfs, under >> + blobs. In case of QEMU, these then appear under domain's sysfs (if the guest >> + kernel has FW_CFG_SYSFS config option enabled), under >> ``/sys/firmware/qemu_fw_cfg``. Note, that these values apply regardless the >> <smbios/> mode under <os/>. :since:`Since 6.5.0` >> >> + **Please note that because of limited number of data slots use of fwcfg is >> + strongly discouraged and <oemStrings/> should be used instead**. > > please replace: > > strongly discouraged > > with: > > strongly discouraged for configuring any guest-side component other > than the firmware > > ( > > Consider for example the following feature: > > https://bugzilla.tianocore.org/show_bug.cgi?id=2681 > > Namely, the following QEMU switches: > > -fw_cfg name=opt/org.tianocore/IPv4PXESupport,string=[yn] > -fw_cfg name=opt/org.tianocore/IPv6PXESupport,string=[yn] > > alter the behavior of OVMF and ArmVirtQemu. These flags are meant to be > stable. They do not need dedicated QEMU or libvirtd enablement. They > influence firmware behavior. So <sysinfo type='fwcfg'> is perfectly fine > (even ideal!) for tweaking them, through the domain XML. What's not fine > is configuring any random guest payload via <sysinfo type='fwcfg'>. > > The point is that people who parse new fw_cfg files in edk2 such as > "opt/org.tianocore/IPv6PXESupport" are conscious of the slot count in > QEMU. They *can* bump the "x-file-slots" property in QEMU, for new > machine types, they just need to be aware of the property. I'd suggest that QEMU always sets x-file-slots to be 10 entries larger than the number of officially known slots. That leads some scope for app usage without risk of hitting the limits.

>> - <entry name='opt/com.coreos/config' file='/tmp/provision.ign'/> >> - </smbios> >> + <entry name='opt/com.example/config' file='/tmp/provision.ign'/> > > We have a functional -- working, stable -- example for name+file as > well: > > - name: etc/edk2/https/cacerts > - file: /etc/pki/ca-trust/extracted/edk2/cacerts.bin I don't think we should document that in libvirt, since it is something libvirt will set silently behind the scenes. The ignition example is a acceptable real world example of expected usage in libvirt.