Re: [dpdk-dev] [PATCH v5 17/19] vhost: restrict postcopy live-migration enablement

[Tiwei Bie <tiwei.bie@intel.com>]

On Tue, Oct 09, 2018 at 10:54:24PM +0200, Maxime Coquelin wrote:
> Postcopy live-migration feature requires the application to
> not populate the guest memory. As the vhost library cannot
> prevent the application to that (e.g. preventing the
> application to call mlockall()), the feature is disabled by
> default.
> 
> The application should only enable the feature if it does not
> force the guest memory to be populated.
> 
> In case the user passes the RTE_VHOST_USER_POSTCOPY_SUPPORT
> flag at registration but the feature was not compiled,
> registration fails.
> 
> For the same reason, postcopy and dequeue zero copy features
> are not compatible, so don't advertize postcopy support if
> dequeue zero copy is requested.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  doc/guides/prog_guide/vhost_lib.rst |  8 ++++++++
>  lib/librte_vhost/rte_vhost.h        |  1 +
>  lib/librte_vhost/socket.c           | 30 ++++++++++++++++++++++++++---
>  3 files changed, 36 insertions(+), 3 deletions(-)
> 
> diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
> index 77af4d775..c77df338f 100644
> --- a/doc/guides/prog_guide/vhost_lib.rst
> +++ b/doc/guides/prog_guide/vhost_lib.rst
> @@ -106,6 +106,14 @@ The following is an overview of some key Vhost API functions:
>      Enabling this flag with these Qemu version results in Qemu being blocked
>      when multiple queue pairs are declared.
>  
> +  - ``RTE_VHOST_USER_POSTCOPY_SUPPORT``
> +
> +    Postcopy live-migration support will be enabled when this flag is set.
> +    It is disabled by default.
> +
> +    Enabling this flag should only be done when the calling application does
> +    not pre-fault the guest shared memory, otherwise migration would fail.
> +
>  * ``rte_vhost_driver_set_features(path, features)``
>  
>    This function sets the feature bits the vhost-user driver supports. The
> diff --git a/lib/librte_vhost/rte_vhost.h b/lib/librte_vhost/rte_vhost.h
> index 9292c89c5..d280ac420 100644
> --- a/lib/librte_vhost/rte_vhost.h
> +++ b/lib/librte_vhost/rte_vhost.h
> @@ -28,6 +28,7 @@ extern "C" {
>  #define RTE_VHOST_USER_NO_RECONNECT	(1ULL << 1)
>  #define RTE_VHOST_USER_DEQUEUE_ZERO_COPY	(1ULL << 2)
>  #define RTE_VHOST_USER_IOMMU_SUPPORT	(1ULL << 3)
> +#define RTE_VHOST_USER_POSTCOPY_SUPPORT		(1ULL << 4)
>  
>  /** Protocol features. */
>  #ifndef VHOST_USER_PROTOCOL_F_MQ
> diff --git a/lib/librte_vhost/socket.c b/lib/librte_vhost/socket.c
> index 7cad5593e..4b221a805 100644
> --- a/lib/librte_vhost/socket.c
> +++ b/lib/librte_vhost/socket.c
> @@ -51,6 +51,8 @@ struct vhost_user_socket {
>  	uint64_t supported_features;
>  	uint64_t features;
>  
> +	uint64_t protocol_features;

In vhost_user_set_protocol_features() in vhost_user.c,
we also need to find a way to check with this field
instead of VHOST_USER_PROTOCOL_FEATURES when receiving
the protocol features from QEMU.

> +
>  	/*
>  	 * Device id to identify a specific backend device.
>  	 * It's set to -1 for the default software implementation.
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