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    SubjectRe: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers
    Il 18/12/2012 14:36, Michael S. Tsirkin ha scritto:
    > Some comments without arguing about whether the performance
    > benefit is worth it.
    >
    > On Tue, Dec 18, 2012 at 01:32:48PM +0100, Paolo Bonzini wrote:
    >> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
    >> index cf8adb1..39d56c4 100644
    >> --- a/include/linux/virtio.h
    >> +++ b/include/linux/virtio.h
    >> @@ -7,6 +7,7 @@
    >> #include <linux/spinlock.h>
    >> #include <linux/device.h>
    >> #include <linux/mod_devicetable.h>
    >> +#include <linux/dma-direction.h>
    >> #include <linux/gfp.h>
    >>
    >> /**
    >> @@ -40,6 +41,26 @@ int virtqueue_add_buf(struct virtqueue *vq,
    >> void *data,
    >> gfp_t gfp);
    >>
    >> +struct virtqueue_buf {
    >> + struct virtqueue *vq;
    >> + struct vring_desc *indirect, *tail;
    >
    > This is wrong: virtio.h does not include virito_ring.h,
    > and it shouldn't by design depend on it.
    >
    >> + int head;
    >> +};
    >> +
    >
    > Can't we track state internally to the virtqueue?
    > Exposing it seems to buy us nothing since you can't
    > call add_buf between start and end anyway.

    I wanted to keep the state for these functions separate from the rest.
    I don't think it makes much sense to move it to struct virtqueue unless
    virtqueue_add_buf is converted to use the new API (doesn't make much
    sense, could even be a tad slower).

    On the other hand moving it there would eliminate the dependency on
    virtio_ring.h. Rusty, what do you think?

    >> +int virtqueue_start_buf(struct virtqueue *_vq,
    >> + struct virtqueue_buf *buf,
    >> + void *data,
    >> + unsigned int count,
    >> + unsigned int count_sg,
    >> + gfp_t gfp);
    >> +
    >> +void virtqueue_add_sg(struct virtqueue_buf *buf,
    >> + struct scatterlist sgl[],
    >> + unsigned int count,
    >> + enum dma_data_direction dir);
    >> +
    >
    > And idea: in practice virtio scsi seems to always call sg_init_one, no?
    > So how about we pass in void* or something and avoid using sg and count?
    > This would make it useful for -net BTW.

    It also passes the scatterlist from the LLD. It calls sg_init_one for
    the request/response headers.

    Paolo

    >> +void virtqueue_end_buf(struct virtqueue_buf *buf);
    >> +
    >> void virtqueue_kick(struct virtqueue *vq);
    >>
    >> bool virtqueue_kick_prepare(struct virtqueue *vq);
    >> --
    >> 1.7.1
    >>



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