netdev - Re: [PATCH V9 mlx5-next 10/15] vfio: Extend the device migration protocol with RUNNING

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Date:   Wed, 02 Mar 2022 12:51:49 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Yishai Hadas <yishaih@...dia.com>, alex.williamson@...hat.com,
        bhelgaas@...gle.com, jgg@...dia.com, saeedm@...dia.com
Cc:     linux-pci@...r.kernel.org, kvm@...r.kernel.org,
        netdev@...r.kernel.org, kuba@...nel.org, leonro@...dia.com,
        kwankhede@...dia.com, mgurtovoy@...dia.com, yishaih@...dia.com,
        maorg@...dia.com, ashok.raj@...el.com, kevin.tian@...el.com,
        shameerali.kolothum.thodi@...wei.com
Subject: Re: [PATCH V9 mlx5-next 10/15] vfio: Extend the device migration
 protocol with RUNNING_P2P

On Thu, Feb 24 2022, Yishai Hadas <yishaih@...dia.com> wrote:

> From: Jason Gunthorpe <jgg@...dia.com>
>
> The RUNNING_P2P state is designed to support multiple devices in the same
> VM that are doing P2P transactions between themselves. When in RUNNING_P2P
> the device must be able to accept incoming P2P transactions but should not
> generate outgoing P2P transactions.
>
> As an optional extension to the mandatory states it is defined as
> inbetween STOP and RUNNING:
>    STOP -> RUNNING_P2P -> RUNNING -> RUNNING_P2P -> STOP
>
> For drivers that are unable to support RUNNING_P2P the core code
> silently merges RUNNING_P2P and RUNNING together. Unless driver support
> is present, the new state cannot be used in SET_STATE.
> Drivers that support this will be required to implement 4 FSM arcs
> beyond the basic FSM. 2 of the basic FSM arcs become combination
> transitions.
>
> Compared to the v1 clarification, NDMA is redefined into FSM states and is
> described in terms of the desired P2P quiescent behavior, noting that
> halting all DMA is an acceptable implementation.
>
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>
> Reviewed-by: Kevin Tian <kevin.tian@...el.com>
> Signed-off-by: Yishai Hadas <yishaih@...dia.com>
> ---
>  drivers/vfio/vfio.c       | 84 +++++++++++++++++++++++++++++++--------
>  include/linux/vfio.h      |  1 +
>  include/uapi/linux/vfio.h | 36 ++++++++++++++++-
>  3 files changed, 102 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index b37ab27b511f..bdb5205bb358 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -1577,39 +1577,55 @@ int vfio_mig_get_next_state(struct vfio_device *device,
>  			    enum vfio_device_mig_state new_fsm,
>  			    enum vfio_device_mig_state *next_fsm)
>  {
> -	enum { VFIO_DEVICE_NUM_STATES = VFIO_DEVICE_STATE_RESUMING + 1 };
> +	enum { VFIO_DEVICE_NUM_STATES = VFIO_DEVICE_STATE_RUNNING_P2P + 1 };
>  	/*
> -	 * The coding in this table requires the driver to implement 6
> +	 * The coding in this table requires the driver to implement
>  	 * FSM arcs:

Nit: this now reads a bit strange; maybe "requires the driver to
implement the following FSM arcs"?

>  	 *         RESUMING -> STOP
> -	 *         RUNNING -> STOP
>  	 *         STOP -> RESUMING
> -	 *         STOP -> RUNNING
>  	 *         STOP -> STOP_COPY
>  	 *         STOP_COPY -> STOP
>  	 *
> -	 * The coding will step through multiple states for these combination
> -	 * transitions:
> -	 *         RESUMING -> STOP -> RUNNING
> +	 * If P2P is supported then the driver must also implement these FSM
> +	 * arcs:
> +	 *         RUNNING -> RUNNING_P2P
> +	 *         RUNNING_P2P -> RUNNING
> +	 *         RUNNING_P2P -> STOP
> +	 *         STOP -> RUNNING_P2P
> +	 * Without P2P the driver must implement:
> +	 *         RUNNING -> STOP
> +	 *         STOP -> RUNNING
> +	 *
> +	 * If all optional features are supported then the coding will step
> +	 * through multiple states for these combination transitions:

Maybe "The coding will step through multiple states for some combination
transitions; if all optional features are supported, this means the
following ones:"?

> +	 *         RESUMING -> STOP -> RUNNING_P2P
> +	 *         RESUMING -> STOP -> RUNNING_P2P -> RUNNING
>  	 *         RESUMING -> STOP -> STOP_COPY
> -	 *         RUNNING -> STOP -> RESUMING
> -	 *         RUNNING -> STOP -> STOP_COPY
> +	 *         RUNNING -> RUNNING_P2P -> STOP
> +	 *         RUNNING -> RUNNING_P2P -> STOP -> RESUMING
> +	 *         RUNNING -> RUNNING_P2P -> STOP -> STOP_COPY
> +	 *         RUNNING_P2P -> STOP -> RESUMING
> +	 *         RUNNING_P2P -> STOP -> STOP_COPY
> +	 *         STOP -> RUNNING_P2P -> RUNNING
>  	 *         STOP_COPY -> STOP -> RESUMING
> -	 *         STOP_COPY -> STOP -> RUNNING
> +	 *         STOP_COPY -> STOP -> RUNNING_P2P
> +	 *         STOP_COPY -> STOP -> RUNNING_P2P -> RUNNING
>  	 */