Re: [Xen-devel] [PATCH] libxlu: Handle += in config files

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Fri, Aug 16, 2019 at 12:47:07PM +0000, Wei Liu wrote:
> On Tue, Aug 13, 2019 at 05:42:15PM +0100, Anthony PERARD wrote:
> > On Tue, Aug 13, 2019 at 04:47:23PM +0100, Andrew Cooper wrote:
> > > Error between user and terminal. :)
> > > 
> > > I'd sync'd xl and libxl.so, but not libxlu.so
> > 
> > I actually made the same mistake first time I tried.
> > 
> > > Ok, so that is working now.  I think 'cmdline+=" dom0=pvh
> > > dom0-iommu=none"' is slightly less tortured syntax, but I guess there is
> > > no way that this isn't going to be horrible.
> > > 
> > > As for the general mechanism, how usable is += for anything other than
> > > cmdline?  Most strings in config files can't usefully be extended in
> > > this matter - if they need changing, they need changing wholesale.
> > 
> > That's true, but one could imaging some maybe bad example like adding a
> > suffix to the name of the guest: "name+='-ovmf';".
> > Going through `man xl.cfg', maybe a good example other than cmdline
> > could be "cpus+=',^1'" but maybe a space is fine here, or one could use
> > a list instead.
> > Other potential uses could be for "PATH", but in this case it would be
> > better reset the setting rather that attempting to add a suffix to an
> > existing one.
> > 
> > I wonder if instead of doing += on all strings, we should instead have
> > `xl' whitelist the few options where += would make sense. (and at that
> > point, it would be easy to add a ' ' where is make sense, like
> > "cmdline"s. But then, how to tell users that it can't do "name+='-new'"?
> > because xlu would just print a warning, and xl would keep going with
> > name="".  Try "xl create memory+=42" ;-).
> 
> Not sure I follow. Can you limit this in xl?

With a patch to xl, yes ;-). What could be done is to add a new API in
libxlu. The original function get_string() could return an error if +=
was used on a specific setting. A new function get_appended_string()
(working title), could return return a string with the += computation
done. That way, xl can choose which setting are allowed to have += used
(and it could even use how to do it, i.e. add spaces between strings or
not).

> Isn't += handled in libxlu already?

It's not, this patch adds the capability into libxlu.

Cheers,

-- 
Anthony PERARD

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