Re: [Qemu-devel] [RFC 26/29] vhost: Add VHOST_USER_POSTCOPY_END message

[Maxime Coquelin]

On 06/28/2017 09:00 PM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
>
> This message is sent just before the end of postcopy to get the
> client to stop using userfault since we wont respond to any more
> requests.  It should close userfaultfd so that any other pages
> get mapped to the backing file automatically by the kernel, since
> at this point we know we've received everything.
>
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> ---
>   contrib/libvhost-user/libvhost-user.c | 23 +++++++++++++++++++++++
>   contrib/libvhost-user/libvhost-user.h |  1 +
>   hw/virtio/vhost-user.c                |  1 +
>   3 files changed, 25 insertions(+)
>
> diff --git a/contrib/libvhost-user/libvhost-user.c 
> b/contrib/libvhost-user/libvhost-user.c
> index d37052b7b0..c1716d1a62 100644
> --- a/contrib/libvhost-user/libvhost-user.c
> +++ b/contrib/libvhost-user/libvhost-user.c
> @@ -68,6 +68,7 @@ vu_request_to_string(int req)
>           REQ(VHOST_USER_INPUT_GET_CONFIG),
>           REQ(VHOST_USER_POSTCOPY_ADVISE),
>           REQ(VHOST_USER_POSTCOPY_LISTEN),
> +        REQ(VHOST_USER_POSTCOPY_END),
>           REQ(VHOST_USER_MAX),
>       };
>   #undef REQ
> @@ -889,6 +890,26 @@ vu_set_postcopy_listen(VuDev *dev, VhostUserMsg *vmsg)
>   
>       return false;
>   }
> +
> +static bool
> +vu_set_postcopy_end(VuDev *dev, VhostUserMsg *vmsg)
> +{
> +    fprintf(stderr, "%s: Entry\n", __func__);
> +    dev->postcopy_listening = false;
> +    if (dev->postcopy_ufd > 0) {
> +        close(dev->postcopy_ufd);
> +        dev->postcopy_ufd = -1;
> +        fprintf(stderr, "%s: Done close\n", __func__);
> +    }
> +
> +    vmsg->fd_num = 0;
> +    vmsg->payload.u64 = 0;
> +    vmsg->size = sizeof(vmsg->payload.u64);
> +    vmsg->flags = VHOST_USER_VERSION |  VHOST_USER_REPLY_MASK;
> +    fprintf(stderr, "%s: exit\n", __func__);
> +    return true;
> +}
> +

It is what reply-ack is done for, so to avoid code duplication,
maybe Qemu could set VHOST_USER_NEED_REPLY_MASK bit when reply-ack
feature is supported.

I'm wondering if we shouldn't consider adding reply-ack feature support
to libvhost-user, and make postcopy support to depend on this feature.

Cheers,
Maxime