Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy)

[Eric Blake <eblake@redhat.com>]

On Sat, Oct 09, 2021 at 04:56:13AM -0300, Leonardo Bras wrote:
> Implement zerocopy on nocomp_send_write(), by making use of QIOChannel
> zerocopy interface.
> 
> Change multifd_send_sync_main() so it can distinguish the last sync from
> the setup and per-iteration ones, so a flush_zerocopy() can be called
> at the last sync in order to make sure all RAM is sent before finishing
> the migration.
> 
> Also make it return -1 if flush_zerocopy() fails, in order to cancel
> the migration process, and avoid resuming the guest in the target host
> without receiving all current RAM.
> 
> This will work fine on RAM migration because the RAM pages are not usually freed,
> and there is no problem on changing the pages content between async_send() and
> the actual sending of the buffer, because this change will dirty the page and
> cause it to be re-sent on a next iteration anyway.
> 
> Given a lot of locked memory may be needed in order to use multid migration
> with zerocopy enabled, make it optional by creating a new parameter
> multifd-zerocopy on qapi, so low-privileged users can still perform multifd
> migrations.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  qapi/migration.json   | 18 ++++++++++++++++++
>  migration/migration.h |  1 +
>  migration/multifd.h   |  2 +-
>  migration/migration.c | 20 ++++++++++++++++++++
>  migration/multifd.c   | 33 ++++++++++++++++++++++++++++-----
>  migration/ram.c       | 20 +++++++++++++-------
>  monitor/hmp-cmds.c    |  4 ++++
>  7 files changed, 85 insertions(+), 13 deletions(-)
> 
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 88f07baedd..c4890cbb54 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -724,6 +724,11 @@
>  #                      will consume more CPU.
>  #                      Defaults to 1. (Since 5.0)
>  #
> +# @multifd-zerocopy: Controls behavior on sending memory pages on multifd migration.
> +#                    When true, enables a zerocopy mechanism for sending memory
> +#                    pages, if host does support it.

s/does support/supports/ (several instances this patch)

> +#                    Defaults to false. (Since 6.2)
> +#
>  # @block-bitmap-mapping: Maps block nodes and bitmaps on them to
>  #                        aliases for the purpose of dirty bitmap migration.  Such
>  #                        aliases may for example be the corresponding names on the
> @@ -758,6 +763,7 @@
>             'xbzrle-cache-size', 'max-postcopy-bandwidth',
>             'max-cpu-throttle', 'multifd-compression',
>             'multifd-zlib-level' ,'multifd-zstd-level',
> +	   'multifd-zerocopy',
>             'block-bitmap-mapping' ] }

Should this feature be guarded with 'if':'CONFIG_LINUX', since that's
the only platform where you even compile the code (even then, it can
still fail if the kernel doesn't support it).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org