CVS commit: src/sys/arch/xen/xen

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Sat Feb 25 00:32:49 UTC 2023

Modified Files:
        src/sys/arch/xen/xen: if_xennet_xenbus.c

Log Message:
xennet(4): Membar audit.

- xennet_tx_complete: Other side owns rsp_prod, giving us responses
  to tx commands.  We own rsp_cons, recording which responess we've
  processed already.

  1. Other side initializes responses before advancing rsp_prod, so
     we must observe rsp_prod before trying to examine the responses.
     Hence load from rsp_prod must be followed by xen_rmb.

     (Can this just use atomic_load_acquire?)

  2. As soon as other side observes rsp_event, it may start to
     overwrite now-unused response slots, so we must finish using the
     response before advancing rsp_cons.  Hence we must issue xen_wmb
     before store to rsp_event.

     (Can this just use atomic_store_release?)

     (Should this use RING_FINAL_CHECK_FOR_RESPONSES?)

  3. When loop is done and we set rsp_event, we must ensure the other
     side has had a chance to see that we want more before we check
     whether there is more to consume; otherwise the other side might
     not bother to send us an interrupt.  Hence after setting
     rsp_event, we must issue xen_mb (store-before-load) before
     re-checking rsp_prod.

- xennet_handler (rx): Same deal, except the xen_mb is buried in
  RING_FINAL_CHECK_FOR_RESPONSES.  Unclear why xennet_tx_complete has
  this open-coded while xennet_handler (rx) uses the macro.

XXX pullup-8 (at least the xen_mb part; requires patch)
XXX pullup-9 (at least the xen_mb part; requires patch)
XXX pullup-10


To generate a diff of this commit:
cvs rdiff -u -r1.128 -r1.129 src/sys/arch/xen/xen/if_xennet_xenbus.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.