Re: [Qemu-devel] [PATCH v4] migration: do not rom_reset() during incoming migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

Paolo, can you take this one please.

* Catherine Ho (catherine.hecx@gmail.com) wrote:
> Commit 18269069c310 ("migration: Introduce ignore-shared capability")
> addes ignore-shared capability to bypass the shared ramblock (e,g,
> membackend + numa node). It does good to live migration.
> 
> As told by Yury,this commit expectes that QEMU doesn't write to guest RAM
> until VM starts, but it does on aarch64 qemu:
> Backtrace:
> 1  0x000055f4a296dd84 in address_space_write_rom_internal () at
> exec.c:3458
> 2  0x000055f4a296de3a in address_space_write_rom () at exec.c:3479
> 3  0x000055f4a2d519ff in rom_reset () at hw/core/loader.c:1101
> 4  0x000055f4a2d475ec in qemu_devices_reset () at hw/core/reset.c:69
> 5  0x000055f4a2c90a28 in qemu_system_reset () at vl.c:1675
> 6  0x000055f4a2c9851d in main () at vl.c:4552
> 
> Actually, on arm64 virt marchine, ramblock "dtb" will be filled into ram
> druing rom_reset. In ignore-shared incoming case, this rom filling
> is not required since all the data has been stored in memory backend
> file.
> 
> Further more, as suggested by Peter Xu, if we do rom_reset() now with
> these ROMs then the RAM data should be re-filled again too with the
> migration stream coming in.
> 
> Fixes: commit 18269069c310 ("migration: Introduce ignore-shared
> capability")
> Suggested-by: Yury Kotov <yury-kotov@yandex-team.ru>
> Suggested-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Catherine Ho <catherine.hecx@gmail.com>
> ---
>  hw/core/loader.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/hw/core/loader.c b/hw/core/loader.c
> index fe5cb24122..040109464b 100644
> --- a/hw/core/loader.c
> +++ b/hw/core/loader.c
> @@ -1087,6 +1087,15 @@ static void rom_reset(void *unused)
>  {
>      Rom *rom;
>  
> +    /*
> +     * 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))
> +        return;
> +
>      QTAILQ_FOREACH(rom, &roms, next) {
>          if (rom->fw_file) {
>              continue;
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK