Re: [Qemu-devel] [PATCH v2] Sort the fw_cfg file list

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 15, 2016 at 07:38:43AM -0500, Corey Minyard wrote:
> On 03/15/2016 04:37 AM, Michael S. Tsirkin wrote:
> >On Tue, Mar 15, 2016 at 09:45:22AM +0100, Gerd Hoffmann wrote:
> >>>Depends on how you code it up.  We have a list, we look each file
> >>>there and sort accordingly.  Fine.
> >>>New devices will not be on this list, I guess you can just ignore them
> >>>and guests will not see them. OK but I think it is better to make old
> >>>machine types see them.
> >>Not a new fw_cfg file.
> >>
> >>It's existing smbios file which gets new records added by a new device.
> >>So when initializing it early (old order) it doesn't (yet) contain the
> >>new records.  When initializing it late it has them, but also has a
> >>different place in the fw_cfg directory.
> >>
> >>So old machine types initialize smbios early (for compatibility).
> >I see. So in this model, we'd have to somehow keep track of
> >the old initialization order forever, and
> >add hacks whenever we change it.
> >IMHO That would just be too hard to maintain. I have an alternative
> >proposal.
> >
> >
> >
> >>New machine types initialize smbios late (so guests see the new
> >>records).
> >
> >So here is what I propose instead:
> >
> >- always initialize it late
> >- sort late, a machine done, not when inserting entries
> >- figure out what the order of existing entries is currently,
> >   and fill an array listing them in this order.
> >   for old machine types, insert the existing entries
> >   in this specific order by using a sorting function:
> >
> >qsort(....., fw_cfg_cmp);
> >
> >where:
> >
> >fw_cfg_find(a) {
> >     for (index = 0; index < fw_cfg_legacy_array_size; ++index)
> >         if (!strcmp(a, ...))
> >             break;
> >     return index;
> >}
> >
> >fw_cfg_cmp(a, b) {
> >     in cmp;
> >     if (legacy_fw_cfg_order) {
> >         int list1 = find(a);
> >         int list2 = find(b);
> >
> >         if (list1 < list2)
> >             return -1;
> >         if (list1 > list2)
> >             return 1;
> >     }
> >
> >     return strcmp(a, b);
> >}
> 
> Last night I had an idea something like this.  Sorting by filename
> may not work because the user may pass in the file from the
> command line and you wouldn't be able to track the file name that
> way.

command line files must all have a consistent prefix,
so we can skip sorting them.
I'll need to look at the code - don't they already?
If not we IMHO absolutely must fix that before release
and give them consistent prefixes.

> Instead, you could add a "legacy_order" parameter to the fw_cfg_add
> functions.  Then figure out the current order add the numeric
> order to each call.  Then sort by the numeric order.  As long as you
> don't reorder things with the same numeric value I think that
> would work and be fairly simple to implement.  New calls could
> pass in NO_FW_CFG_LEGACY_ORDER or something like that and
> be pasted onto the end in legacy mode.
> 
> -corey

OK but it's a much larger change and less well contained.

> >
> >
> >
> >
> >
> >
> >>While mucking with the file ordering anyway:  Good opportunity to make
> >>new machine types also sort the fw_cfg directory entries, so they get a
> >>fixed order independent from the order they are created, and we will not
> >>face this problem again.
> >>
> >>cheers,
> >>   Gerd
> >What exactly do you mean by directory entries here?
> >