Linux Virtualization - Jun 2013 - [PATCH RFC] Adapter interrupts for virtio-ccw.

Hi,

here's a proposal to support adapter (aka thin) interrupts for virtio-ccw.

The basic idea is to make host->guest signalling on s390 more lightweight.
Normal I/O interrupts have two parts: an interrupt that is made pending on
any of the guest cpus, and status that is made pending on the subchannel.
This means that we need two exits for every interrupt.

With adapter interrupts, only the I/O interrupt remains - no status is made
pending for the subchannel. To find out which virtqueue the signal was for
we rely on indicators.

To set this up, the guest uses a new ccw (which is used instead of the
normal ccw to set up indicators). The payload contains pointers to two
indicators (first level and second level) and the offset at which the
virtqueue indicators start in the second level indicators, as well as the
interruption subclass (which will usually be the same as the isc for the
device). This is partially inspired by what qdio does today.

I have seen some nice speedup on simple dd with my current implementation.
Adapter interrupts are also a prereq for implementing irqfd on s390, since
they eliminate the need for manipulating subchannel status.

Cornelia Huck (1):
  virtio-ccw: Document adapter interrupts.

 virtio-spec.lyx |  147 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 144 insertions(+), 3 deletions(-)

-- 
1.7.9.5

Signed-off-by: Cornelia Huck <cornelia.huck at de.ibm.com>
---
 virtio-spec.lyx |  147 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 144 insertions(+), 3 deletions(-)

diff --git a/virtio-spec.lyx b/virtio-spec.lyx
index 6e188d0..697351e 100644
--- a/virtio-spec.lyx
+++ b/virtio-spec.lyx
@@ -10701,11 +10701,18 @@ status open
 
 \begin_layout LyX-Code
 
-\change_inserted -385801441 1343732726
+\change_inserted -385801441 1369814105
 
 #define CCW_CMD_READ_VQ_CONF 0x32
 \end_layout
 
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369814140
+
+#define CCW_CMD_SET_IND_ADAPTER 0x63
+\end_layout
+
 \end_inset
 
 
@@ -11045,11 +11052,136 @@ To communicate the location of the indicator bits for
host->guest notification,
 
 \begin_layout Standard
 
-\change_inserted -385801441 1347015749
+\change_inserted -385801441 1369814376
 For the indicator bits used in the configuration change host->guest
notification
 , the CCW_CMD_SET_CONF_IND command is used analogously.
 \end_layout
 
+\begin_layout Subsubsection*
+
+\change_inserted -385801441 1369814399
+Setting Up Indicators For Adapter Interrupts
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted -385801441 1369815013
+If the guest wishes to use adapter interrupts for host->guest notification,
+ it may use the CCW_CMD_SET_IND_ADAPTER command instead of CCW_CMD_SET_IND.
+ Note that usage of those two mechanisms is mutually exclusive.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted -385801441 1369815065
+CCW_CMD_SET_IND_ADAPTER uses the following communication block:
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369815367
+\begin_inset listings
+inline false
+status open
+
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369815367
+
+struct thinint_area {
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369815367
+
+        unsigned long summary_indicator;
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369815367
+
+        unsigned long indicator;
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369815367
+
+        u16 shift;
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369815367
+
+        u8 isc;
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted -385801441 1369815367
+
+} __packed;
+\change_unchanged
+
+\end_layout
+
+\end_inset
+
+
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted -385801441 1370345028
+
+\family typewriter
+summary_indicator
+\family default
+ contains the guest address of a byte value to be used as a summary indicator
+ which is set to != 0 every time the host wants to signal the guest for
+ any of the indictors and unset by the guest to signify that it received
+ the notification.
+ 
+\family typewriter
+isc
+\family default
+ is the interruption subclass to be used for the adapter interrupt.
+ Note that an isc/summary indicator pair must match for any subsequent requests
+ to set up adapter interrupts .
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted -385801441 1369816401
+
+\family typewriter
+indicator
+\family default
+ contains the guest address of the 64 bit indicators to be used; 
+\family typewriter
+shift
+\family default
+ contains the offset of the queue indicators for the device in this value.
+ All queue indicators for a device must fit into the same 64 bit value.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted -385801441 1369814707
+Hosts not supporting adapter interrupts for virtio-ccw may fail this command
+ with a command reject.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted -385801441 1369814766
+Configuration change host->guest notification is always setup using
CCW_CMD_SET_
+CONF_IND.
+\end_layout
+
 \begin_layout Subsection*
 
 \change_inserted -385801441 1343732726
@@ -11064,7 +11196,7 @@ Host->Guest Notification
 
 \begin_layout Standard
 
-\change_inserted -385801441 1347015762
+\change_inserted -385801441 1369814838
 For notifying the guest of virtqueue buffers, the host sets the corresponding
  bit in the guest-provided indicators.
  If an interrupt is not already pending for the subchannel, the host generates
@@ -11073,6 +11205,15 @@ For notifying the guest of virtqueue buffers, the host
sets the corresponding
 
 \begin_layout Standard
 
+\change_inserted -385801441 1369815397
+Alternatively, if the guest enabled adapter interrupts for a device,
notificatio
+n happens via setting the bit in the guest-provided indicators, setting
+ the summary indicator and generating an adapter interrupt for the registered
+ interruption subclass.
+\end_layout
+
+\begin_layout Standard
+
 \change_inserted -385801441 1347015847
 If the host wants to notify the guest about configuration changes, it sets
  bit 0 in the configuration indicators and generates an unsolicited I/O
-- 
1.7.9.5