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Subject[PATCH] tools/virtio: add inorder option
skips ring accesses but drops out of order support
At high level this follows alf design
https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/alf_queue.c
but on most (single-threaded) benchmarks this seems to be slower than the layout
without index use.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
tools/virtio/ringtest/virtio_ring_0_9.c | 49 ++++++++++++++++++++++++++++-
tools/virtio/ringtest/virtio_ring_inorder.c | 2 ++
tools/virtio/ringtest/Makefile | 4 ++-
3 files changed, 53 insertions(+), 2 deletions(-)
create mode 100644 tools/virtio/ringtest/virtio_ring_inorder.c

diff --git a/tools/virtio/ringtest/virtio_ring_0_9.c b/tools/virtio/ringtest/virtio_ring_0_9.c
index 47c9a1a..7618662 100644
--- a/tools/virtio/ringtest/virtio_ring_0_9.c
+++ b/tools/virtio/ringtest/virtio_ring_0_9.c
@@ -26,6 +26,14 @@ struct vring ring;
* high bits of ring id ^ 0x8000).
*/
/* #ifdef RING_POLL */
+/* enabling the below activates experimental in-order code
+ * (which skips ring updates and reads and writes len in descriptor).
+ */
+/* #ifdef INORDER */
+
+#if defined(RING_POLL) && defined(INORDER)
+#error "RING_POLL and INORDER are mutually exclusive"
+#endif

/* how much padding is needed to avoid false cache sharing */
#define HOST_GUEST_PADDING 0x80
@@ -35,7 +43,11 @@ struct guest {
unsigned short last_used_idx;
unsigned short num_free;
unsigned short kicked_avail_idx;
+#ifndef INORDER
unsigned short free_head;
+#else
+ unsigned short reserved_free_head;
+#endif
unsigned char reserved[HOST_GUEST_PADDING - 10];
} guest;

@@ -66,8 +78,10 @@ void alloc_ring(void)
guest.avail_idx = 0;
guest.kicked_avail_idx = -1;
guest.last_used_idx = 0;
+#ifndef INORDER
/* Put everything in free lists. */
guest.free_head = 0;
+#endif
for (i = 0; i < ring_size - 1; i++)
ring.desc[i].next = i + 1;
host.used_idx = 0;
@@ -84,13 +98,20 @@ void alloc_ring(void)
/* guest side */
int add_inbuf(unsigned len, void *buf, void *datap)
{
- unsigned head, avail;
+ unsigned head;
+#ifndef INORDER
+ unsigned avail;
+#endif
struct vring_desc *desc;

if (!guest.num_free)
return -1;

+#ifdef INORDER
+ head = (ring_size - 1) & (guest.avail_idx++);
+#else
head = guest.free_head;
+#endif
guest.num_free--;

desc = ring.desc;
@@ -102,7 +123,9 @@ int add_inbuf(unsigned len, void *buf, void *datap)
* descriptors.
*/
desc[head].flags &= ~VRING_DESC_F_NEXT;
+#ifndef INORDER
guest.free_head = desc[head].next;
+#endif

data[head].data = datap;

@@ -113,8 +136,12 @@ int add_inbuf(unsigned len, void *buf, void *datap)
ring.avail->ring[avail & (ring_size - 1)] =
(head | (avail & ~(ring_size - 1))) ^ 0x8000;
#else
+#ifndef INORDER
+ /* Barrier A (for pairing) */
+ smp_release();
avail = (ring_size - 1) & (guest.avail_idx++);
ring.avail->ring[avail] = head;
+#endif
/* Barrier A (for pairing) */
smp_release();
#endif
@@ -141,15 +168,27 @@ void *get_buf(unsigned *lenp, void **bufp)
return NULL;
/* Barrier B (for pairing) */
smp_acquire();
+#ifdef INORDER
+ head = (ring_size - 1) & guest.last_used_idx;
+ index = head;
+#else
head = (ring_size - 1) & guest.last_used_idx;
index = ring.used->ring[head].id;
#endif
+
+#endif
+#ifdef INORDER
+ *lenp = ring.desc[index].len;
+#else
*lenp = ring.used->ring[head].len;
+#endif
datap = data[index].data;
*bufp = (void*)(unsigned long)ring.desc[index].addr;
data[index].data = NULL;
+#ifndef INORDER
ring.desc[index].next = guest.free_head;
guest.free_head = index;
+#endif
guest.num_free++;
guest.last_used_idx++;
return datap;
@@ -283,16 +322,24 @@ bool use_buf(unsigned *lenp, void **bufp)
smp_acquire();

used_idx &= ring_size - 1;
+#ifdef INORDER
+ head = used_idx;
+#else
head = ring.avail->ring[used_idx];
+#endif
desc = &ring.desc[head];
#endif

*lenp = desc->len;
*bufp = (void *)(unsigned long)desc->addr;

+#ifdef INORDER
+ desc->len = desc->len - 1;
+#else
/* now update used ring */
ring.used->ring[used_idx].id = head;
ring.used->ring[used_idx].len = desc->len - 1;
+#endif
/* Barrier B (for pairing) */
smp_release();
host.used_idx++;
diff --git a/tools/virtio/ringtest/virtio_ring_inorder.c b/tools/virtio/ringtest/virtio_ring_inorder.c
new file mode 100644
index 0000000..2438ca5
--- /dev/null
+++ b/tools/virtio/ringtest/virtio_ring_inorder.c
@@ -0,0 +1,2 @@
+#define INORDER 1
+#include "virtio_ring_0_9.c"
diff --git a/tools/virtio/ringtest/Makefile b/tools/virtio/ringtest/Makefile
index feaa64a..8037971 100644
--- a/tools/virtio/ringtest/Makefile
+++ b/tools/virtio/ringtest/Makefile
@@ -1,6 +1,6 @@
all:

-all: ring virtio_ring_0_9 virtio_ring_poll
+all: ring virtio_ring_0_9 virtio_ring_poll virtio_ring_inorder

CFLAGS += -Wall
CFLAGS += -pthread -O2 -ggdb
@@ -13,10 +13,12 @@ virtio_ring_poll.o: virtio_ring_poll.c virtio_ring_0_9.c main.h
ring: ring.o main.o
virtio_ring_0_9: virtio_ring_0_9.o main.o
virtio_ring_poll: virtio_ring_poll.o main.o
+virtio_ring_inorder: virtio_ring_inorder.o main.o
clean:
-rm main.o
-rm ring.o ring
-rm virtio_ring_0_9.o virtio_ring_0_9
-rm virtio_ring_poll.o virtio_ring_poll
+ -rm virtio_ring_inorder.o virtio_ring_inorder

.PHONY: all clean
--
MST
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