[Qemu-devel] virtio-s390: document GPR4/GPR2 cookie values

[Michael S. Tsirkin]

virtio-s390 on kvm can use a cookie value passed to guest
to optimize channel/VQ lookups.
Document this.

Signed-off-by: Michael S. Tsirkin <address@hidden>

---
diff --git a/virtio-spec.lyx b/virtio-spec.lyx
index 72d956c..91aed06 100644
--- a/virtio-spec.lyx
+++ b/virtio-spec.lyx
@@ -10627,7 +10626,252 @@ Guest->Host Notification
 For notifying the host of virtqueue buffers, the guest unfortunately can't
  use a channel command (the asynchronous characteristics of channel I/O
  interact badly with the host block I/O backend).
- Instead, it uses a diagnose 0x500 call with subcode 3 specifying the queue.
+ Instead, it uses a diagnose 0x500 call with subcode 3 specifying the queue
+\change_inserted 1986246365 1362677938
+, as follows:
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1362677973
+\begin_inset Tabular
+<lyxtabular version="3" rows="5" columns="3">
+<features tabularvalignment="middle">
+<column alignment="center" valignment="top" width="0">
+<column alignment="center" valignment="top" width="0">
+<column alignment="center" valignment="top" width="0">
+<row>
+<cell alignment="center" valignment="top" topline="true" bottomline="true" 
leftline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362677978
+GPR
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" bottomline="true" 
leftline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678629
+Input Value
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" bottomline="true" 
leftline="true" rightline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678627
+Output Value
+\end_layout
+
+\end_inset
+</cell>
+</row>
+<row>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362677991
+1
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678208
+0x3
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
rightline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678620
+
+\end_layout
+
+\end_inset
+</cell>
+</row>
+<row>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678226
+2
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678498
+Subchannel ID
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
rightline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678642
+Host Cookie
+\end_layout
+
+\end_inset
+</cell>
+</row>
+<row>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678500
+3
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678540
+Virtqueue number
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" leftline="true" 
rightline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678620
+
+\end_layout
+
+\end_inset
+</cell>
+</row>
+<row>
+<cell alignment="center" valignment="top" topline="true" bottomline="true" 
leftline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678529
+4
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" bottomline="true" 
leftline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678644
+Host Cookie
+\change_unchanged
+
+\end_layout
+
+\end_inset
+</cell>
+<cell alignment="center" valignment="top" topline="true" bottomline="true" 
leftline="true" rightline="true" usebox="none">
+\begin_inset Text
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362678620
+
+\end_layout
+
+\end_inset
+</cell>
+</row>
+</lyxtabular>
+
+\end_inset
+
+
+\change_deleted 1986246365 1362677938
+.
+\change_inserted 1986246365 1362678646
+
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1362679010
+Host cookie is an optional per-virtqueue 64 bit value that can be used by
+ the hypervisor to speed up the notification execution.
+ For each notification, the output value is returned in GPR2 and should
+ be passed in GPR4 for the next notification:
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted 1986246365 1362679011
+\begin_inset listings
+inline false
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1362679033
+
+info->cookie = do_notify(schid, virtqueue_get_queue_index(vq), info->cookie);
+\end_layout
+
+\end_inset
+
+
 \end_layout
 
 \begin_layout Subsubsection*