Re: [Qemu-devel] [PATCH v5 1/6] vhost-user: Support transferring inflight buffer between qemu and backend

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Jan 22, 2019 at 04:31:47PM +0800, elohimes@gmail.com wrote:
> +typedef struct DescState {
> +    uint8_t inuse;
> +    uint8_t version;
> +    uint16_t used_idx;
> +    uint16_t avail_idx;
> +    uint16_t reserved;
> +} DescState;
> +
> +typedef struct QueueRegion {
> +    uint8_t valid;
> +    uint16_t desc_num;
> +    DescState desc[0];
> +} QueueRegion;
> +
> +The struct DescState is used to describe one head-descriptor's state. The
> +fields have following meanings:
> +
> +    inuse: Indicate whether the descriptor is inuse or not.
> +
> +    version: Indicate whether we have an atomic update to used ring and
> +    inflight buffer when slave crash at that point. This field should be
> +    increased by one before and after this two updates. An odd version
> +    indicates an in-progress update.
> +
> +    used_idx: Store old index of used ring before we update used ring and
> +    inflight buffer so that slave can know whether an odd version inflight
> +    head-descriptor in inflight buffer is processed or not.
> +
> +    avail_idx: Used to preserve the descriptor's order in avail ring so that
> +    slave can resubmit descriptors in order.

Will a completely new "packed vring" inflight shm layout be necessary to
support the packed vring layout in VIRTIO 1.1?

https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-610007

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