Re: [Qemu-devel] [PATCH] migration: incoming postcopy advise sanity checks

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

06.02.2018 10:26, Greg Kurz wrote:
> If postcopy-ram was set on the source but not on the destination,
> migration doesn't occur, the destination prints an error and boots
> the guest:
>
> qemu-system-ppc64: Expected vmdescription section, but got 0
>
> We end up with two running instances.
>
> This behaviour was introduced in 2.11 by commit 58110f0acb1a "migration:
> split common postcopy out of ram postcopy" to prepare ground for the
> upcoming dirty bitmap postcopy support. It adds a new case where the
> source may send an empty postcopy advise because dirty bitmap doesn't
> need to check page sizes like RAM postcopy does.
>
> If the source has enabled postcopy-ram, then it sends an advise with
> the page size values. If the destination hasn't enabled postcopy-ram,
> then loadvm_postcopy_handle_advise() leaves the page size values on
> the stream and returns. This confuses qemu_loadvm_state() later on
> and causes the destination to start execution.
>
> As discussed several times, postcopy-ram should be enabled both sides
> to be functional. This patch changes the destination to perform some
> extra checks on the advise length to ensure this is the case. Otherwise
> an error is returned and migration is aborted.
>
> Reported-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>   migration/savevm.c |   18 +++++++++++++++---
>   1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/migration/savevm.c b/migration/savevm.c
> index b7908f62be3c..1c516fcbb8d7 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1376,7 +1376,8 @@ static int qemu_loadvm_state_main(QEMUFile *f, MigrationIncomingState *mis);
>    * *might* happen - it might be skipped if precopy transferred everything
>    * quickly.
>    */
> -static int loadvm_postcopy_handle_advise(MigrationIncomingState *mis)
> +static int loadvm_postcopy_handle_advise(MigrationIncomingState *mis,
> +                                         uint16_t len)
>   {
>       PostcopyState ps = postcopy_state_set(POSTCOPY_INCOMING_ADVISE);
>       uint64_t remote_pagesize_summary, local_pagesize_summary, remote_tps;
> @@ -1387,8 +1388,19 @@ static int loadvm_postcopy_handle_advise(MigrationIncomingState *mis)
>           return -1;
>       }
>   
> -    if (!migrate_postcopy_ram()) {
> +    switch (len) {
> +    case 0:
> +        /* The source hasn't enabled postcopy-ram. Nothing to do. */

should we error-out here if (migrate_postcopy_ram()) ?

>           return 0;
> +    case 8 + 8:
> +        if (!migrate_postcopy_ram()) {
> +            error_report("RAM postcopy is disabled");
> +            return -EINVAL;
> +        }
> +        break;
> +    default:
> +        error_report("CMD_POSTCOPY_ADVISE invalid length (%d)", len);
> +        return -EINVAL;
>       }
>   
>       if (!postcopy_ram_supported_by_host(mis)) {
> @@ -1807,7 +1819,7 @@ static int loadvm_process_command(QEMUFile *f)
>           return loadvm_handle_cmd_packaged(mis);
>   
>       case MIG_CMD_POSTCOPY_ADVISE:
> -        return loadvm_postcopy_handle_advise(mis);
> +        return loadvm_postcopy_handle_advise(mis, len);
>   
>       case MIG_CMD_POSTCOPY_LISTEN:
>           return loadvm_postcopy_handle_listen(mis);
>


-- 
Best regards,
Vladimir