Re: [PATCH v1 1/1] migration: Avoid false-positive on non-supported scenarios for zero-copy-send

[Peter Xu]

On Tue, Jul 19, 2022 at 09:23:45AM -0300, Leonardo Bras wrote:
> Migration with zero-copy-send currently has it's limitations, as it can't
> be used with TLS nor any kind of compression. In such scenarios, it should
> output errors during parameter / capability setting.
> 
> But currently there are some ways of setting this not-supported scenarios
> without printing the error message:
> 
> !) For 'compression' capability, it works by enabling it together with
> zero-copy-send. This happens because the validity test for zero-copy uses
> the helper unction migrate_use_compression(), which check for compression
> presence in s->enabled_capabilities[MIGRATION_CAPABILITY_COMPRESS].
> 
> The point here is: the validity test happens before the capability gets
> enabled. If all of them get enabled together, this test will not return
> error.
> 
> In order to fix that, replace migrate_use_compression() by directly testing
> the cap_list parameter migrate_caps_check().
> 
> 2) For features enabled by parameters such as TLS & 'multifd_compression',
> there was also a possibility of setting non-supported scenarios: setting
> zero-copy-send first, then setting the unsupported parameter.
> 
> In order to fix that, also add a check for parameters conflicting with
> zero-copy-send on migrate_params_check().
> 
> 3) XBZRLE is also a compression capability, so it makes sense to also add
> it to the list of capabilities which are not supported with zero-copy-send.
> 
> Fixes: 1abaec9a1b2c ("migration: Change zero_copy_send from migration 
> parameter to migration capability")
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  migration/migration.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 78f5057373..c6260e54bf 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1274,7 +1274,9 @@ static bool migrate_caps_check(bool *cap_list,
>  #ifdef CONFIG_LINUX
>      if (cap_list[MIGRATION_CAPABILITY_ZERO_COPY_SEND] &&
>          (!cap_list[MIGRATION_CAPABILITY_MULTIFD] ||
> -         migrate_use_compression() ||
> +         cap_list[MIGRATION_CAPABILITY_COMPRESS] ||
> +         cap_list[MIGRATION_CAPABILITY_XBZRLE] ||
> +         migrate_multifd_compression() ||
>           migrate_use_tls())) {
>          error_setg(errp,
>                     "Zero copy only available for non-compressed non-TLS 
> multifd migration");
> @@ -1511,6 +1513,17 @@ static bool migrate_params_check(MigrationParameters 
> *params, Error **errp)
>          error_prepend(errp, "Invalid mapping given for block-bitmap-mapping: 
> ");
>          return false;
>      }
> +
> +#ifdef CONFIG_LINUX

A trivial nit: we don't need this since migrate_use_zero_copy_send() will
be defined unconditionally, and it's returning false with !CONFIG_LINUX.
So feel free to drop this if there's a new version.

> +    if (migrate_use_zero_copy_send() &&
> +        ((params->has_multifd_compression && params->multifd_compression) ||
> +         (params->has_tls_creds && params->tls_creds && 
> *params->tls_creds))) {
> +        error_setg(errp,
> +                   "Zero copy only available for non-compressed non-TLS 
> multifd migration");
> +        return false;
> +    }
> +#endif

Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks,

-- 
Peter Xu