[PATCH] docs: Discourage users from using fwcfg

[Laszlo Ersek]

On 09/08/20 08:37, Martin Kletzander wrote:
> On Mon, Sep 07, 2020 at 03:38:23PM +0100, Daniel P. Berrangé wrote:
>> On Mon, Sep 07, 2020 at 04:20:02PM +0200, Michal Privoznik wrote:
>>> On 9/7/20 3:57 PM, Martin Kletzander wrote:
>>> > On Mon, Sep 07, 2020 at 03:48:16PM +0200, 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 at redhat.com>
>>> > > Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>>> > > ---
>>> > > docs/formatdomain.rst | 14 +++++++++-----
>>> > > 1 file changed, 9 insertions(+), 5 deletions(-)
>>> > >
>>>
>>>
>>> > It's nice that you say that, but people who would like to use
>>> fw_cfg for
>>> > passing
>>> > in a huge blob, which is saved in a file, will read this, go to
>>> > <oemStrings/>
>>> > and see that there is no way to pass a file as an input.  Should
>>> that be
>>> > dealt
>>> > with somehow?  Or would that be discouraged as well?
>>>
>>> Unfortunately, QEMU doesn't allow reading OEM strings from a file (at
>>> least
>>> quick glance over hw/smbios/smbios.c doesn't show any signs it's
>>> allowed).
>>
>> Indeed, that is an RFE I've never got around to
>>
> 
> Do you mean RFE for QEMU or libvirt?
> 
> Are there any limitations for the oemStrings?  Libvirt could read the
> file and
> pass the data to QEMU as it is not something that is supposed to be
> writeable or
> shareable.  I understand it could be the first time we do something like
> this,
> but it might not be that bad of a precedence.
> 
> Just so we are on the same page, I'm not against this patch, I just want
> to save
> our users the frustration that I, myself, experienced so many times. 
> There's
> something deep hurting when you finally find exactly what you're looking
> for,
> only to learn that it is deprecated with another option which does not
> provide
> the same functionality.

This description is not entirely accurate. I would put it like this: you
stumble upon a mis-use of a feature that was originally meant for
something else. It seems that the original consumers of the feature have
always been unhappy about the mis-use (it presents a risk for the
subsystem they are responsible for), in spite of the mis-use possibly
scratching your itch too.

That shouldn't hurt your feelings -- the point is that it's not
"deprecation"; the original thing has never been condoned for this
particular creative kind of abuse. Instead, the feature that could
scratch your itch has never existed.

Thanks,
Laszlo


> Sure, you can have a start hook that reads a
> file and
> changes the data, etc.  But you get the point.  At least we could
> mention that?
> 
>> Regards,
>> Daniel
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