Subject: Re: [PATCH net-next V2 0/8] Packed virtqueue support for vhost

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 16 Jul 2018 11:39:11 +0300

On Mon, Jul 16, 2018 at 11:28:03AM +0800, Jason Wang wrote:
> Hi all:
> 
> This series implements packed virtqueues. The code were tested with
> Tiwei's guest driver series at https://patchwork.ozlabs.org/cover/942297/
> 
> 
> Pktgen test for both RX and TX does not show obvious difference with
> split virtqueues. The main bottleneck is the guest Linux driver, since
> it can not stress vhost for a 100% CPU utilization. A full TCP
> benchmark is ongoing. Will test virtio-net pmd as well when it was
> ready.

Well the question then is why we should bother merging this
if this doesn't give a performance gain. Do you see
a gain in CPU utilization maybe?

If not - let's wait for that TCP benchmark result?

> Notes:
> - This version depends on Tiwei's series at https://patchwork.ozlabs.org/cover/942297/
> 
> This version were tested with:
> 
> - Zerocopy (Out of Order) support
> - vIOMMU support
> - mergeable buffer on/off
> - busy polling on/off
> - vsock (nc-vsock)
> 
> Changes from V1:
> - drop uapi patch and use Tiwei's
> - split the enablement of packed virtqueue into a separate patch
> 
> Changes from RFC V5:
> 
> - save unnecessary barriers during vhost_add_used_packed_n()
> - more compact math for event idx
> - fix failure of SET_VRING_BASE when avail_wrap_counter is true
> - fix not copy avail_wrap_counter during GET_VRING_BASE
> - introduce SET_VRING_USED_BASE/GET_VRING_USED_BASE for syncing last_used_idx
> - rename used_wrap_counter to last_used_wrap_counter
> - rebase to net-next
> 
> Changes from RFC V4:
> 
> - fix signalled_used index recording
> - track avail index correctly
> - various minor fixes
> 
> Changes from RFC V3:
> 
> - Fix math on event idx checking
> - Sync last avail wrap counter through GET/SET_VRING_BASE
> - remove desc_event prefix in the driver/device structure
> 
> Changes from RFC V2:
> 
> - do not use & in checking desc_event_flags
> - off should be most significant bit
> - remove the workaround of mergeable buffer for dpdk prototype
> - id should be in the last descriptor in the chain
> - keep _F_WRITE for write descriptor when adding used
> - device flags updating should use ADDR_USED type
> - return error on unexpected unavail descriptor in a chain
> - return false in vhost_ve_avail_empty is descriptor is available
> - track last seen avail_wrap_counter
> - correctly examine available descriptor in get_indirect_packed()
> - vhost_idx_diff should return u16 instead of bool
> 
> Changes from RFC V1:
> 
> - Refactor vhost used elem code to avoid open coding on used elem
> - Event suppression support (compile test only).
> - Indirect descriptor support (compile test only).
> - Zerocopy support.
> - vIOMMU support.
> - SCSI/VSOCK support (compile test only).
> - Fix several bugs
> 
> Jason Wang (8):
>   vhost: move get_rx_bufs to vhost.c
>   vhost: hide used ring layout from device
>   vhost: do not use vring_used_elem
>   vhost_net: do not explicitly manipulate vhost_used_elem
>   vhost: vhost_put_user() can accept metadata type
>   vhost: packed ring support
>   vhost: event suppression for packed ring
>   vhost: enable packed virtqueues
> 
>  drivers/vhost/net.c        | 143 ++-----
>  drivers/vhost/scsi.c       |  62 +--
>  drivers/vhost/vhost.c      | 994 ++++++++++++++++++++++++++++++++++++++++-----
>  drivers/vhost/vhost.h      |  55 ++-
>  drivers/vhost/vsock.c      |  42 +-
>  include/uapi/linux/vhost.h |   7 +
>  6 files changed, 1035 insertions(+), 268 deletions(-)
> 
> -- 
> 2.7.4