Re: [PATCH] exec/rom_reset: Free rom data during inmigrate skip

[Peter Maydell <peter.maydell@linaro.org>]

On Fri, 13 Mar 2020 at 12:31, Dr. David Alan Gilbert (git)
<dgilbert@redhat.com> wrote:
>
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>
> Commit 355477f8c73e9 skips rom reset when we're an incoming migration
> so as not to overwrite shared ram in the ignore-shared migration
> optimisation.
> However, it's got an unexpected side effect that because it skips
> freeing the ROM data, when rom_reset gets called later on, after
> migration (e.g. during a reboot), the ROM does get reset to the original
> file contents.  Because of seabios/x86's weird reboot process
> this confuses a reboot into hanging after a migration.
>
> Fixes: 355477f8c73e9 ("migration: do not rom_reset() during incoming migration")
> https://bugzilla.redhat.com/show_bug.cgi?id=1809380
>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  hw/core/loader.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
>

>      QTAILQ_FOREACH(rom, &roms, next) {
>          if (rom->fw_file) {
>              continue;
>          }
> +        /*
> +         * We don't need to fill in the RAM with ROM data because we'll fill
> +         * the data in during the next incoming migration in all cases.  Note
> +         * that some of those RAMs can actually be modified by the guest on ARM
> +         * so this is probably the only right thing to do here.
> +         */
> +        if (runstate_check(RUN_STATE_INMIGRATE) && rom->data) {
> +            /*
> +             * Free it so that a rom_reset after migration doesn't overwrite a
> +             * potentially modified 'rom'.
> +             */
> +            rom_free_data(rom);

Shouldn't this condition match the condition in rom_reset()
for when we call rom_free_data()? You want the behaviour
on a subsequent reset to match the behaviour you'd get
if you did a reset on the source end without the migration.

> +        }
> +
>          if (rom->data == NULL) {
>              continue;
>          }

If you put this check above your new one you wouldn't
need to check rom->data in it. Also it would make the
loop structure better match rom_reset(), which checks
rom->fw_file first, then rom->data, then has the
condition for "do we need to call rom_free_data()".

thanks
-- PMM