Re: [PATCH 2/8] migration: Add precopy initial data handshake

[Juan Quintela <quintela@redhat.com>]

Avihai Horon <avihaih@nvidia.com> wrote:
> Add precopy initial data handshake between source and destination upon
> migration setup. The purpose of the handshake is to notify the
> destination that precopy initial data is used and which migration users
> (i.e., SaveStateEntry) are going to use it.
>
> The handshake is done in two levels. First, a general enable command is
> sent to notify the destination migration code that precopy initial data
> is used.
> Then, for each migration user in the source that supports precopy
> initial data, an enable command is sent to its counterpart in the
> destination:
> If both support it, precopy initial data will be used for them.
> If source doesn't support it, precopy initial data will not be used for
> them.
> If source supports it and destination doesn't, migration will be failed.
>
> To implement it, a new migration command MIG_CMD_INITIAL_DATA_ENABLE is
> added, as well as a new SaveVMHandlers handler initial_data_advise.
> Calling the handler advises the migration user that precopy initial data
> is used and its return value indicates whether precopy initial data is
> supported by it.
>
> Signed-off-by: Avihai Horon <avihaih@nvidia.com>

> diff --git a/migration/savevm.c b/migration/savevm.c
> index a9181b444b..2740defdf0 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -71,6 +71,13 @@
>  
>  const unsigned int postcopy_ram_discard_version;
>  
> +typedef struct {
> +    uint8_t general_enable;

I miss a comment for this field.

I think that you only use the values 0 and 1
And that 1 means something like: we require this feature and do nothing
And that 0 means that this is a device that uses the feature and
<something, something>

> +    uint8_t reserved[7];
> +    uint8_t idstr[256];
> +    uint32_t instance_id;
> +} InitialDataInfo;

We have several "reserved" space here.  Do we want a Version field?
It don't seem that we need a size field, as the command is fixed length.

> @@ -90,6 +97,8 @@ enum qemu_vm_cmd {
>      MIG_CMD_ENABLE_COLO,       /* Enable COLO */
>      MIG_CMD_POSTCOPY_RESUME,   /* resume postcopy on dest */
>      MIG_CMD_RECV_BITMAP,       /* Request for recved bitmap on dst */
> +

Spurious blank line

> +    MIG_CMD_INITIAL_DATA_ENABLE, /* Enable precopy initial data in dest */
>      MIG_CMD_MAX



> +void qemu_savevm_send_initial_data_enable(MigrationState *ms, QEMUFile *f)
> +{
> +    SaveStateEntry *se;
> +    InitialDataInfo buf;

Small nit.

The new way in the block to declare that something needs to be
initialized to zero is:

    InitialDataInfo buf = {};

And no, I have no clue if this makes the compiler generate any better code.

> +    /* Enable precopy initial data generally in the migration */
> +    memset(&buf, 0, sizeof(buf));
> +    buf.general_enable = 1;
> +    qemu_savevm_command_send(f, MIG_CMD_INITIAL_DATA_ENABLE, sizeof(buf),
> +                             (uint8_t *)&buf);
> +    trace_savevm_send_initial_data_enable(buf.general_enable, (char *)buf.idstr,
> +                                          buf.instance_id);

No buf.idstr here.

Why do we need a command before the loop and seeing if we are having any
device that requires this.

> +    /* Enable precopy initial data for each migration user that supports it */
> +    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
> +        if (!se->ops || !se->ops->initial_data_advise) {
> +            continue;
> +        }
> +
> +        if (!se->ops->initial_data_advise(se->opaque)) {
> +            continue;
> +        }

Is this callback going to send anything?

> +
> +        memset(&buf, 0, sizeof(buf));
> +        memcpy(buf.idstr, se->idstr, sizeof(buf.idstr));
> +        buf.instance_id = se->instance_id;
> +
> +        qemu_savevm_command_send(f, MIG_CMD_INITIAL_DATA_ENABLE, sizeof(buf),
> +                                 (uint8_t *)&buf);
> +        trace_savevm_send_initial_data_enable(
> +            buf.general_enable, (char *)buf.idstr, buf.instance_id);

It is me or general_enable is always zero here?

> +    }
> +}
> +
>  void qemu_savevm_send_colo_enable(QEMUFile *f)
>  {
>      trace_savevm_send_colo_enable();
> @@ -2314,6 +2359,60 @@ static int loadvm_process_enable_colo(MigrationIncomingState *mis)
>      return ret;
>  }
>  
> +static int loadvm_handle_initial_data_enable(MigrationIncomingState *mis)
> +{
> +    InitialDataInfo buf;
> +    SaveStateEntry *se;
> +    ssize_t read_size;
> +
> +    read_size = qemu_get_buffer(mis->from_src_file, (void *)&buf, sizeof(buf));
> +    if (read_size != sizeof(buf)) {
> +        error_report("%s: Could not get data buffer, read_size %ld, len %lu",
> +                     __func__, read_size, sizeof(buf));
> +
> +        return -EIO;
> +    }
> +
> +    /* Enable precopy initial data generally in the migration */
> +    if (buf.general_enable) {
> +        mis->initial_data_enabled = true;
> +        trace_loadvm_handle_initial_data_enable(
> +            buf.general_enable, (char *)buf.idstr, buf.instance_id);
> +
> +        return 0;
> +    }
> +
> +    /* Enable precopy initial data for a specific migration user */
> +    se = find_se((char *)buf.idstr, buf.instance_id);
> +    if (!se) {
> +        error_report("%s: Could not find SaveStateEntry, idstr '%s', "
> +                     "instance_id %" PRIu32,
> +                     __func__, buf.idstr, buf.instance_id);
> +
> +        return -ENOENT;
> +    }
> +
> +    if (!se->ops || !se->ops->initial_data_advise) {
> +        error_report("%s: '%s' doesn't have required "
> +                     "initial_data_advise op",
> +                     __func__, buf.idstr);
> +
> +        return -EOPNOTSUPP;
> +    }
> +
> +    if (!se->ops->initial_data_advise(se->opaque)) {
> +        error_report("%s: '%s' doesn't support precopy initial data", __func__,
> +                     buf.idstr);

This is not your fault.  Just venting.

And here we are, again, with a place where we can't return errors.  Sniff.

> +
> +        return -EOPNOTSUPP;
> +    }

I have to wait until I see the usage later in the series, but it is a
good idea to have a single handle for source and destination, and not
passing at least a parameter telling where are we?

Really nice patch, very good done and very good integrated with the
surrounded style.  A pleasure.

Later, Juan.