Re: [Qemu-devel] [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG

[Wei Wang]

On 07/12/2017 09:56 PM, Michael S. Tsirkin wrote:
> So the way I see it, there are several issues:
>
> - internal wait - forces multiple APIs like kick/kick_sync
>    note how kick_sync can fail but your code never checks return code
> - need to re-write the last descriptor - might not work
>    for alternative layouts which always expose descriptors
>    immediately

Probably it wasn't clear. Please let me explain the two functions here:

1) virtqueue_add_chain_desc(vq, head_id, prev_id,..):
grabs a desc from the vq and inserts it to the chain tail (which is 
indexed by
prev_id, probably better to call it tail_id). Then, the new added desc 
becomes
the tail (i.e. the last desc). The _F_NEXT flag is cleared for each desc 
when it's
added to the chain, and set when another desc comes to follow later.

2) virtqueue_add_chain(vq, head_id,..): expose the chain to the other end.

So, if people want to add a desc and immediately expose it to the other end,
i.e. build a single desc chain, they can just add and expose:

virtqueue_add_chain_desc(..);
virtqueue_add_chain(..,head_id);

Would you see any issues here?


> - some kind of iterator type would be nicer instead of
>    maintaining head/prev explicitly

Why would we need to iterate the chain? I think it would be simpler to use
a wrapper struct:

struct virtqueue_desc_chain {
    unsigned int head;  // head desc id of the chain
    unsigned int tail;     // tail desc id of the chain
}

The new desc will be put to desc[tail].next, and we don't need to walk
from the head desc[head].next when inserting a new desc to the chain, right?


> As for the use, it would be better to do
>
> if (!add_next(vq, ...)) {
>         add_last(vq, ...)
>         kick
>         wait
> }

"!add_next(vq, ...)" means that the vq is full? If so, what would 
add_last() do then?


> Using VIRTQUEUE_DESC_ID_INIT seems to avoid a branch in the driver, but
> in fact it merely puts the branch in the virtio code.

Actually it wasn't intended to improve performance. It is used to 
indicate the "init" state
of the chain. So, when virtqueue_add_chain_desc(, head_id,..) finds head 
id=INIT, it will
assign the grabbed desc id to &head_id. In some sense, it is equivalent 
to add_first().

Do you have a different opinion here?

Best,
Wei