Re: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for packed vring desc avail flags

["Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>]

Hi Marvin,

Thanks for your answers, one more question for x86:
1. For CIO memory alone or MMIO memory(eg PCI BAR) alone, the compiler barrier is enough to keep ordering, that's why both rte_io_mb and rte_cio_mb are defined as compiler barriers, right?
2. How about the ordering of interleaved CIO and MMIO accesses, for example, a young store to MMIO can be reordered before an older store to CIO? CIO may be faster than devices, but store buffers or caching may cause the CIO update not visible to the device(in a common doorbell case)? 

Best regards,
Gavin

> -----Original Message-----
> From: Liu, Yong <yong.liu@intel.com>
> Sent: Wednesday, September 11, 2019 10:39 AM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>; Wang, Yinan
> <yinan.wang@intel.com>; Maxime Coquelin <maxime.coquelin@redhat.com>;
> Joyce Kong (Arm Technology China) <Joyce.Kong@arm.com>; dev@dpdk.org
> Cc: nd <nd@arm.com>; Bie, Tiwei <tiwei.bie@intel.com>; Wang, Zhihong
> <zhihong.wang@intel.com>; amorenoz@redhat.com; Wang, Xiao W
> <xiao.w.wang@intel.com>; jfreimann@redhat.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Steve Capper <Steve.Capper@arm.com>
> Subject: RE: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for packed vring
> desc avail flags
> 
> 
> 
> > -----Original Message-----
> > From: Gavin Hu (Arm Technology China) [mailto:Gavin.Hu@arm.com]
> > Sent: Tuesday, September 10, 2019 5:49 PM
> > To: Wang, Yinan <yinan.wang@intel.com>; Maxime Coquelin
> > <maxime.coquelin@redhat.com>; Joyce Kong (Arm Technology China)
> > <Joyce.Kong@arm.com>; dev@dpdk.org
> > Cc: nd <nd@arm.com>; Bie, Tiwei <tiwei.bie@intel.com>; Wang, Zhihong
> > <zhihong.wang@intel.com>; amorenoz@redhat.com; Wang, Xiao W
> > <xiao.w.wang@intel.com>; Liu, Yong <yong.liu@intel.com>;
> > jfreimann@redhat.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>;
> > Steve Capper <Steve.Capper@arm.com>
> > Subject: RE: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for packed
> > vring desc avail flags
> >
> > Hi Yinan,
> >
> > We have done a comparative analysis and found with the old code the
> > if(weak_barriers) and else branches were saved on x86 as rte_smp_wmb
> and
> > rte_cio_wmb are identical.
> > http://git.dpdk.org/dpdk/tree/drivers/net/virtio/virtqueue.h#n49
> > For the new code, with Joyce's patches applied, the branches were not
> saved,
> > which requir additional cpu cycles, this caused slight degradation on x86.
> >
> > The patches uplifted the performance on aarch64 about 9% as indicated in
> > the cover letter. While I am thinking over a solution to the degradation on
> > x86,could you help answer:
> > 1. Is rte_cio_wmb is sufficient for the non weak-barrier case(HW
> > offloading)?
> >  I got this question because I see in Intel NIC PMDs, it is almost never
> > used, it is rte_wmb that is more widely used to notify the NIC device, any
> > difference between the virtio ring compatible smartNIC device(or vDPA?)
> and
> > i40e like devices?
> 
> Hi Gavin,
> X86 architecture can guarantee that young store happen later than old store.
> So rte_cio_wmb is just compiler memory barrier in x86.
> 
> I think compiler barrier is also enough in pmd, rte_wmb is in pmd because of
> it was inherit from first implementation :)
> 
> Thanks,
> Marvin
> 
> > 2. If the rte_cio_wmb is not sufficient for this case and replaced by
> > stronger barriers, like sfence,  then the branches will not be saved by the
> > compiler, then the problem becomes with the correct use of barriers, other
> > than the degradation.
> >
> > Any comments are welcome!
> >
> > Best Regards,
> > Gavin
> >
> > > -----Original Message-----
> > > From: Wang, Yinan <yinan.wang@intel.com>
> > > Sent: Tuesday, September 10, 2019 11:54 AM
> > > To: Maxime Coquelin <maxime.coquelin@redhat.com>; Joyce Kong (Arm
> > > Technology China) <Joyce.Kong@arm.com>; dev@dpdk.org
> > > Cc: nd <nd@arm.com>; Bie, Tiwei <tiwei.bie@intel.com>; Wang, Zhihong
> > > <zhihong.wang@intel.com>; amorenoz@redhat.com; Wang, Xiao W
> > > <xiao.w.wang@intel.com>; Liu, Yong <yong.liu@intel.com>;
> > > jfreimann@redhat.com; Honnappa Nagarahalli
> > > <Honnappa.Nagarahalli@arm.com>; Gavin Hu (Arm Technology China)
> > > <Gavin.Hu@arm.com>
> > > Subject: RE: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for packed
> > vring
> > > desc avail flags
> > >
> > >
> > > Hi Joyce,
> > >
> > > I just test performance impact of your patch set with code base commit id:
> > > d03d8622db48918d14bfe805641b1766ecc40088, after applying your v3
> patch
> > > set , seven paths of vhost/virtio pvp test shows performance drop as
> > below:
> > >
> > > PVP vhost/virtio 1c1q test	         before apply patch	apply patch
> > > test_perf_pvp_inorder_mergeable     	 7.603	           7.474
> > > test_perf_pvp_inorder_no_mergeable	     7.642	           7.525
> > > test_perf_pvp_mergeable	              7.556	           7.431
> > > test_perf_pvp_normal	                   7.554	           7.478
> > > test_perf_pvp_vector_rx	               7.581	           7.469
> > > test_perf_pvp_virtio11_mergeable	           7.068	           6.905
> > > test_perf_pvp_virtio11_normal	           7.088	           6.888
> > >
> > > Thanks,
> > > Yinan
> > >
> > > > -----Original Message-----
> > > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Maxime
> Coquelin
> > > > Sent: 2019年9月9日 18:10
> > > > To: Joyce Kong <joyce.kong@arm.com>; dev@dpdk.org
> > > > Cc: nd@arm.com; Bie, Tiwei <tiwei.bie@intel.com>; Wang, Zhihong
> > > > <zhihong.wang@intel.com>; amorenoz@redhat.com; Wang, Xiao W
> > > > <xiao.w.wang@intel.com>; Liu, Yong <yong.liu@intel.com>;
> > > > jfreimann@redhat.com; honnappa.nagarahalli@arm.com;
> > > gavin.hu@arm.com
> > > > Subject: Re: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for
> > packed
> > > vring
> > > > desc avail flags
> > > >
> > > >
> > > >
> > > > On 9/9/19 11:14 AM, Joyce Kong wrote:
> > > > > In case VIRTIO_F_ORDER_PLATFORM(36) is not negotiated, then the
> > > > > frontend and backend are assumed to be implemented in software,
> that
> > > > > is they can run on identical CPUs in an SMP configuration.
> > > > > Thus a weak form of memory barriers like rte_smp_r/wmb, other than
> > > > > rte_cio_r/wmb, is sufficient for this case(vq->hw->weak_barriers == 1)
> > > > > and yields better performance.
> > > > > For the above case, this patch helps yielding even better performance
> > > > > by replacing the two-way barriers with C11 one-way barriers for avail
> > > > > flags in packed ring.
> > > > >
> > > > > Meanwhile, a read barrier is required to ensure ordering between
> > > > > descriptor's flags and content reads[1]. With C11, load-acquire can
> > > > > enforce the ordering instead of rmb barrier.
> > > > >
> > > > > [1]https://patchwork.dpdk.org/patch/49109/
> > > > >
> > > > > Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> > > > > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > > > > Reviewed-by: Phil Yang <phil.yang@arm.com>
> > > > > ---
> > > > >  drivers/net/virtio/virtio_rxtx.c                 | 13 +++++++------
> > > > >  drivers/net/virtio/virtio_user/virtio_user_dev.c |  6 +++++-
> > > > >  drivers/net/virtio/virtqueue.h                   | 11 +++++++++++
> > > > >  lib/librte_vhost/vhost.h                         |  2 +-
> > > > >  lib/librte_vhost/virtio_net.c                    | 11 +++++------
> > > > >  5 files changed, 29 insertions(+), 14 deletions(-)
> > > >
> > > > Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > > >
> > > > Thanks,
> > > > Maxime