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

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH v2] Sort the fw_cfg file list
Date: Tue, 15 Mar 2016 11:37:06 +0200

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

> 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);


fw_cfg_find(a) {
    for (index = 0; index < fw_cfg_legacy_array_size; ++index)
        if (!strcmp(a, ...))
    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);

> 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?


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