> -----Original Message-----
> From: KY Srinivasan
> Sent: Sunday, February 11, 2018 5:14 PM
--- snip ---
> > if (rbi->ring_buffer->feature_bits.feat_pending_send_sz) {
> > u32 pending_sz = READ_ONCE(rbi->ring_buffer-
> > >pending_send_sz);
> >
> > /*
> > + * Ensure the read of write_index in
> > hv_get_bytes_to_write()
> > + * happens after the read of pending_send_sz.
> > + */
> > + virt_rmb();
> We can avoid the read barrier by making the initialization of curr_write_sz conditional
> on pending_send_sz being non-zero. Indeed you can make all the signaling code conditional
> on pending_send_sz being non-zero.
>
>
I agree that we can immediately test pending_send_sz for zero, and exit
if zero. A zero value will be by far the most common path, and would
save executing the read barrier and hv_get_bytes_to_write(). Can also
move the calculation of "delta" to after the test for zero.
But I believe the read barrier is still needed on the path where
pending_send_sz is non-zero. Just the testing the value of pending_send_sz
doesn't guarantee that the write_index wouldn't be speculatively read,
and potentially out-of-order. And we have to consider the out-of-order
behaviors on ARM64 as well.
>
> > + curr_write_sz = hv_get_bytes_to_write(rbi);
> > +
> > + /*
> > * If there was space before we began iteration,
> > * then host was not blocked. Also handles case where
> > * pending_sz is zero then host has nothing pending
> > * and does not need to be signaled.
> > */
> > - if (orig_write_sz > pending_sz)
> > + if (curr_write_sz - delta > pending_sz)
> > return;
> >
> > /* If pending write will not fit, don't give false hope. */
> > - if (hv_get_bytes_to_write(rbi) < pending_sz)
> > + if (curr_write_sz <= pending_sz)
> > return;
> > +
> > + vmbus_setevent(channel);
> > }
> >
> > - vmbus_setevent(channel);
> > }
> > EXPORT_SYMBOL_GPL(hv_pkt_iter_close);
> > --
> > 1.8.3.1
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