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



On Tue, Sep 08, 2020 at 08:37:03AM +0200, 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 redhat com>
> > > > > Signed-off-by: Michal Privoznik <mprivozn 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?

Both.

> 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.

The problem is the size of the command line you end up with. You need to have
QMEU support for reading from the file.

> 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.  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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