Subject: Re: [PATCH net-next v3 00/13] virtio: support packed ring

From: Tiwei Bie <[email-masked]>
Date: Wed, 21 Nov 2018 20:42:33 +0800

On Wed, Nov 21, 2018 at 07:20:27AM -0500, Michael S. Tsirkin wrote:
> On Wed, Nov 21, 2018 at 06:03:17PM +0800, Tiwei Bie wrote:
> > Hi,
> > 
> > This patch set implements packed ring support in virtio driver.
> > 
> > A performance test between pktgen (pktgen_sample03_burst_single_flow.sh)
> > and DPDK vhost (testpmd/rxonly/vhost-PMD) has been done, I saw
> > ~30% performance gain in packed ring in this case.
> 
> Thanks a lot, this is very exciting!
> Dave, given the holiday, attempts to wrap up the 1.1 spec and the
> patchset size I would very much appreciate a bit more time for
> review. Say until Nov 28?
> 
> > To make this patch set work with below patch set for vhost,
> > some hacks are needed to set the _F_NEXT flag in indirect
> > descriptors (this should be fixed in vhost):
> > 
> > https://lkml.org/lkml/2018/7/3/33
> 
> Could you pls clarify - do you mean it doesn't yet work with vhost
> because of a vhost bug, and to test it with the linked patches
> you had to hack in _F_NEXT? Because I do not see _F_NEXT
> in indirect descriptors in this patch (which is fine).
> Or did I miss it?

You didn't miss anything. :)

I think it's a small bug in vhost, which Jason may fix very
quickly, so I didn't post it. Below is the hack I used:

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index cd7e755484e3..42faea7d8cf8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -980,6 +980,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
 	unsigned int i, n, err_idx;
 	u16 head, id;
 	dma_addr_t addr;
+	int c = 0;
 
 	head = vq->packed.next_avail_idx;
 	desc = alloc_indirect_packed(total_sg, gfp);
@@ -1001,8 +1002,9 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
 			if (vring_mapping_error(vq, addr))
 				goto unmap_release;
 
-			desc[i].flags = cpu_to_le16(n < out_sgs ?
-						0 : VRING_DESC_F_WRITE);
+			desc[i].flags = cpu_to_le16((n < out_sgs ?
+						0 : VRING_DESC_F_WRITE) |
+				    (++c == total_sg ? 0 : VRING_DESC_F_NEXT));
 			desc[i].addr = cpu_to_le64(addr);
 			desc[i].len = cpu_to_le32(sg->length);
 			i++;
-- 
2.14.1

> 
> > v2 -> v3:
> > - Use leXX instead of virtioXX (MST);
> > - Refactor split ring first (MST);
> > - Add debug helpers (MST);
> > - Put split/packed ring specific fields in sub structures (MST);
> > - Handle normal descriptors and indirect descriptors differently (MST);
> > - Track the DMA addr/len related info in a separate structure (MST);
> > - Calculate AVAIL/USED flags only when wrap counter wraps (MST);
> > - Define a struct/union to read event structure (MST);
> > - Define a macro for wrap counter bit in uapi (MST);
> > - Define the AVAIL/USED bits as shifts instead of values (MST);
> > - s/_F_/_FLAG_/ in VRING_PACKED_EVENT_* as they are values (MST);
> > - Drop the notify workaround for QEMU's tx-timer in packed ring (MST);
> > 
> > v1 -> v2:
> > - Use READ_ONCE() to read event off_wrap and flags together (Jason);
> > - Add comments related to ccw (Jason);
> > 
> > RFC v6 -> v1:
> > - Avoid extra virtio_wmb() in virtqueue_enable_cb_delayed_packed()
> >   when event idx is off (Jason);
> > - Fix bufs calculation in virtqueue_enable_cb_delayed_packed() (Jason);
> > - Test the state of the desc at used_idx instead of last_used_idx
> >   in virtqueue_enable_cb_delayed_packed() (Jason);
> > - Save wrap counter (as part of queue state) in the return value
> >   of virtqueue_enable_cb_prepare_packed();
> > - Refine the packed ring definitions in uapi;
> > - Rebase on the net-next tree;
> > 
> > RFC v5 -> RFC v6:
> > - Avoid tracking addr/len/flags when DMA API isn't used (MST/Jason);
> > - Define wrap counter as bool (Jason);
> > - Use ALIGN() in vring_init_packed() (Jason);
> > - Avoid using pointer to track `next` in detach_buf_packed() (Jason);
> > - Add comments for barriers (Jason);
> > - Don't enable RING_PACKED on ccw for now (noticed by Jason);
> > - Refine the memory barrier in virtqueue_poll();
> > - Add a missing memory barrier in virtqueue_enable_cb_delayed_packed();
> > - Remove the hacks in virtqueue_enable_cb_prepare_packed();
> > 
> > RFC v4 -> RFC v5:
> > - Save DMA addr, etc in desc state (Jason);
> > - Track used wrap counter;
> > 
> > RFC v3 -> RFC v4:
> > - Make ID allocation support out-of-order (Jason);
> > - Various fixes for EVENT_IDX support;
> > 
> > RFC v2 -> RFC v3:
> > - Split into small patches (Jason);
> > - Add helper virtqueue_use_indirect() (Jason);
> > - Just set id for the last descriptor of a list (Jason);
> > - Calculate the prev in virtqueue_add_packed() (Jason);
> > - Fix/improve desc suppression code (Jason/MST);
> > - Refine the code layout for XXX_split/packed and wrappers (MST);
> > - Fix the comments and API in uapi (MST);
> > - Remove the BUG_ON() for indirect (Jason);
> > - Some other refinements and bug fixes;
> > 
> > RFC v1 -> RFC v2:
> > - Add indirect descriptor support - compile test only;
> > - Add event suppression supprt - compile test only;
> > - Move vring_packed_init() out of uapi (Jason, MST);
> > - Merge two loops into one in virtqueue_add_packed() (Jason);
> > - Split vring_unmap_one() for packed ring and split ring (Jason);
> > - Avoid using '%' operator (Jason);
> > - Rename free_head -> next_avail_idx (Jason);
> > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> > - Some other refinements and bug fixes;
> > 
> > 
> > Tiwei Bie (13):
> >   virtio: add packed ring types and macros
> >   virtio_ring: add _split suffix for split ring functions
> >   virtio_ring: put split ring functions together
> >   virtio_ring: put split ring fields in a sub struct
> >   virtio_ring: introduce debug helpers
> >   virtio_ring: introduce helper for indirect feature
> >   virtio_ring: allocate desc state for split ring separately
> >   virtio_ring: extract split ring handling from ring creation
> >   virtio_ring: cache whether we will use DMA API
> >   virtio_ring: introduce packed ring support
> >   virtio_ring: leverage event idx in packed ring
> >   virtio_ring: disable packed ring on unsupported transports
> >   virtio_ring: advertize packed ring layout
> > 
> >  drivers/misc/mic/vop/vop_main.c        |   13 +
> >  drivers/remoteproc/remoteproc_virtio.c |   13 +
> >  drivers/s390/virtio/virtio_ccw.c       |   14 +
> >  drivers/virtio/virtio_ring.c           | 1811 +++++++++++++++++++++++++-------
> >  include/uapi/linux/virtio_config.h     |    3 +
> >  include/uapi/linux/virtio_ring.h       |   52 +
> >  6 files changed, 1530 insertions(+), 376 deletions(-)
> > 
> > -- 
> > 2.14.5