[dpdk-dev] [RFC] net/virtio: use real barriers for vDPA

Michael S. Tsirkin mst at redhat.com
Fri Dec 14 16:01:43 CET 2018


On Fri, Dec 14, 2018 at 05:03:43PM +0300, Ilya Maximets wrote:
> On 14.12.2018 15:58, Michael S. Tsirkin wrote:
> > On Fri, Dec 14, 2018 at 03:37:04PM +0300, Ilya Maximets wrote:
> >> SMP barriers are OK for software vhost implementation, but vDPA is a
> >> DMA capable hardware and we have to use at least DMA barriers to
> >> ensure the memory ordering.
> >>
> >> This patch mimics the kernel virtio-net driver in part of choosing
> >> barriers we need.
> >>
> >> Fixes: a3f8150eac6d ("net/ifcvf: add ifcvf vDPA driver")
> >> Cc: stable at dpdk.org
> >>
> >> Signed-off-by: Ilya Maximets <i.maximets at samsung.com>
> > 
> > Instead of this vendor specific hack, can you please
> > use VIRTIO_F_ORDER_PLATFORM?
> 
> Hmm. Yes. You're probably right.
> But VIRTIO_F_ORDER_PLATFORM a.k.a. VIRTIO_F_IO_BARRIER is not implemented
> anywhere at the time. This will require changes to the whole stack (vhost,
> qemu, virtio drivers) to allow the negotiation.

You are changing virtio drivers anyway, don't you?

> Moreover, I guess that it's
> not offered/correctly treated by the current vDPA HW i.e. it allows to work
> with weak barriers (It allowed to do that by the spec, but I don't think that
> it's right).

The idea is that the device (e.g. vdpa backend) can detect that
- platform is weakly ordered
- VIRTIO_F_ORDER_PLATFORM is not negotiated
and then use a software fallback. Slower but safe.

> This vendor specific hack could allow to work right now.
> 
> Actually, I'm not sure if vDPA is usable with current QEMU ?
> Does anybody know ? If it's usable, than having a hack could be acceptable
> for now. If not, of course, we could drop this and implement virtio native
> solution.

I think it's only usable on x86 right now.

> But yes, I agree that virtio native solution should be done anyway regardless
> of having or not having hacks like this.
> 
> Maybe I can split the patch in two:
> 1. Add ability to use real barriers. ('weak_barriers' always 'true')
> 2. Hack to initialize 'weak_barriers' according to subsystem_device_id.
> 
> The first patch will be needed anyway. The second one could be dropped or
> applied and replaced in the future by proper solution with VIRTIO_F_ORDER_PLATFORM.
> 
> Thoughts ?

I'd rather not do 2 at all since otherwise we will just
get more vendors asking us to special-case their hardware.
Exposing a new feature bit is not a lot of work at all.

> One more thing I have concerns about is that vDPA offers support for earlier
> versions of virtio protocol, which looks strange.

I suspect things like BE just don't work. Again x86 is fine.


> > 
> > 
> >> ---
> >>
> >> Sending as RFC, because the patch is completely not tested. I heve no
> >> HW to test the real behaviour. And I actually do not know if the
> >> subsystem_vendor/device_id's are available at the time and has
> >> IFCVF_SUBSYS_* values inside the real guest system.
> >>
> >> One more thing I want to mention that cross net client Live Migration
> >> is actually not possible without any support from the guest driver.
> >> Because migration from the software vhost to vDPA will lead to using
> >> weaker barriers than required.
> >>
> >> The similar change (weak_barriers = false) should be made in the linux
> >> kernel virtio-net driver.
> > 
> > IMHO no, it should use VIRTIO_F_ORDER_PLATFORM.
> 
> Sure. I'm not a fan of hacks too, especially inside kernel.
> 
> > 
> >> Another dirty solution could be to restrict the vDPA support to x86
> >> systems only.
> >>
> >>  drivers/net/virtio/virtio_ethdev.c      |  9 +++++++
> >>  drivers/net/virtio/virtio_pci.h         |  1 +
> >>  drivers/net/virtio/virtio_rxtx.c        | 14 +++++-----
> >>  drivers/net/virtio/virtio_user_ethdev.c |  1 +
> >>  drivers/net/virtio/virtqueue.h          | 35 +++++++++++++++++++++----
> >>  5 files changed, 48 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
> >> index cb2b2e0bf..249b536fd 100644
> >> --- a/drivers/net/virtio/virtio_ethdev.c
> >> +++ b/drivers/net/virtio/virtio_ethdev.c
> >> @@ -1448,6 +1448,9 @@ virtio_configure_intr(struct rte_eth_dev *dev)
> >>  	return 0;
> >>  }
> >>  
> >> +#define IFCVF_SUBSYS_VENDOR_ID	0x8086
> >> +#define IFCVF_SUBSYS_DEVICE_ID	0x001A
> >> +
> >>  /* reset device and renegotiate features if needed */
> >>  static int
> >>  virtio_init_device(struct rte_eth_dev *eth_dev, uint64_t req_features)
> >> @@ -1477,6 +1480,12 @@ virtio_init_device(struct rte_eth_dev *eth_dev, uint64_t req_features)
> >>  	if (!hw->virtio_user_dev) {
> >>  		pci_dev = RTE_ETH_DEV_TO_PCI(eth_dev);
> >>  		rte_eth_copy_pci_info(eth_dev, pci_dev);
> >> +
> >> +		if (pci_dev->id.subsystem_vendor_id == IFCVF_SUBSYS_VENDOR_ID &&
> >> +		    pci_dev->id.subsystem_device_id == IFCVF_SUBSYS_DEVICE_ID)
> >> +			hw->weak_barriers = 0;
> >> +		else
> >> +			hw->weak_barriers = 1;
> >>  	}
> >>  
> >>  	/* If host does not support both status and MSI-X then disable LSC */
> >> diff --git a/drivers/net/virtio/virtio_pci.h b/drivers/net/virtio/virtio_pci.h
> >> index e961a58ca..1f8e719a9 100644
> >> --- a/drivers/net/virtio/virtio_pci.h
> >> +++ b/drivers/net/virtio/virtio_pci.h
> >> @@ -240,6 +240,7 @@ struct virtio_hw {
> >>  	uint8_t     use_simple_rx;
> >>  	uint8_t     use_inorder_rx;
> >>  	uint8_t     use_inorder_tx;
> >> +	uint8_t     weak_barriers;
> >>  	bool        has_tx_offload;
> >>  	bool        has_rx_offload;
> >>  	uint16_t    port_id;
> >> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> >> index cb8f89f18..66195bf47 100644
> >> --- a/drivers/net/virtio/virtio_rxtx.c
> >> +++ b/drivers/net/virtio/virtio_rxtx.c
> >> @@ -906,7 +906,7 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
> >>  
> >>  	nb_used = VIRTQUEUE_NUSED(vq);
> >>  
> >> -	virtio_rmb();
> >> +	virtio_rmb(hw->weak_barriers);
> >>  
> >>  	num = likely(nb_used <= nb_pkts) ? nb_used : nb_pkts;
> >>  	if (unlikely(num > VIRTIO_MBUF_BURST_SZ))
> >> @@ -1017,7 +1017,7 @@ virtio_recv_mergeable_pkts_inorder(void *rx_queue,
> >>  	nb_used = RTE_MIN(nb_used, nb_pkts);
> >>  	nb_used = RTE_MIN(nb_used, VIRTIO_MBUF_BURST_SZ);
> >>  
> >> -	virtio_rmb();
> >> +	virtio_rmb(hw->weak_barriers);
> >>  
> >>  	PMD_RX_LOG(DEBUG, "used:%d", nb_used);
> >>  
> >> @@ -1202,7 +1202,7 @@ virtio_recv_mergeable_pkts(void *rx_queue,
> >>  
> >>  	nb_used = VIRTQUEUE_NUSED(vq);
> >>  
> >> -	virtio_rmb();
> >> +	virtio_rmb(hw->weak_barriers);
> >>  
> >>  	PMD_RX_LOG(DEBUG, "used:%d", nb_used);
> >>  
> >> @@ -1365,7 +1365,7 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
> >>  	PMD_TX_LOG(DEBUG, "%d packets to xmit", nb_pkts);
> >>  	nb_used = VIRTQUEUE_NUSED(vq);
> >>  
> >> -	virtio_rmb();
> >> +	virtio_rmb(hw->weak_barriers);
> >>  	if (likely(nb_used > vq->vq_nentries - vq->vq_free_thresh))
> >>  		virtio_xmit_cleanup(vq, nb_used);
> >>  
> >> @@ -1407,7 +1407,7 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
> >>  		/* Positive value indicates it need free vring descriptors */
> >>  		if (unlikely(need > 0)) {
> >>  			nb_used = VIRTQUEUE_NUSED(vq);
> >> -			virtio_rmb();
> >> +			virtio_rmb(hw->weak_barriers);
> >>  			need = RTE_MIN(need, (int)nb_used);
> >>  
> >>  			virtio_xmit_cleanup(vq, need);
> >> @@ -1463,7 +1463,7 @@ virtio_xmit_pkts_inorder(void *tx_queue,
> >>  	PMD_TX_LOG(DEBUG, "%d packets to xmit", nb_pkts);
> >>  	nb_used = VIRTQUEUE_NUSED(vq);
> >>  
> >> -	virtio_rmb();
> >> +	virtio_rmb(hw->weak_barriers);
> >>  	if (likely(nb_used > vq->vq_nentries - vq->vq_free_thresh))
> >>  		virtio_xmit_cleanup_inorder(vq, nb_used);
> >>  
> >> @@ -1511,7 +1511,7 @@ virtio_xmit_pkts_inorder(void *tx_queue,
> >>  		need = slots - vq->vq_free_cnt;
> >>  		if (unlikely(need > 0)) {
> >>  			nb_used = VIRTQUEUE_NUSED(vq);
> >> -			virtio_rmb();
> >> +			virtio_rmb(hw->weak_barriers);
> >>  			need = RTE_MIN(need, (int)nb_used);
> >>  
> >>  			virtio_xmit_cleanup_inorder(vq, need);
> >> diff --git a/drivers/net/virtio/virtio_user_ethdev.c b/drivers/net/virtio/virtio_user_ethdev.c
> >> index f8791391a..f075774b4 100644
> >> --- a/drivers/net/virtio/virtio_user_ethdev.c
> >> +++ b/drivers/net/virtio/virtio_user_ethdev.c
> >> @@ -434,6 +434,7 @@ virtio_user_eth_dev_alloc(struct rte_vdev_device *vdev)
> >>  	hw->use_simple_rx = 0;
> >>  	hw->use_inorder_rx = 0;
> >>  	hw->use_inorder_tx = 0;
> >> +	hw->weak_barriers = 1;
> >>  	hw->virtio_user_dev = dev;
> >>  	return eth_dev;
> >>  }
> >> diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
> >> index 26518ed98..7bf17d3bf 100644
> >> --- a/drivers/net/virtio/virtqueue.h
> >> +++ b/drivers/net/virtio/virtqueue.h
> >> @@ -19,15 +19,40 @@
> >>  struct rte_mbuf;
> >>  
> >>  /*
> >> - * Per virtio_config.h in Linux.
> >> + * Per virtio_ring.h in Linux.
> >>   *     For virtio_pci on SMP, we don't need to order with respect to MMIO
> >>   *     accesses through relaxed memory I/O windows, so smp_mb() et al are
> >>   *     sufficient.
> >>   *
> >> + *     For using virtio to talk to real devices (eg. vDPA) we do need real
> >> + *     barriers.
> >>   */
> >> -#define virtio_mb()	rte_smp_mb()
> >> -#define virtio_rmb()	rte_smp_rmb()
> >> -#define virtio_wmb()	rte_smp_wmb()
> >> +static inline void
> >> +virtio_mb(uint8_t weak_barriers)
> >> +{
> >> +	if (weak_barriers)
> >> +		rte_smp_mb();
> >> +	else
> >> +		rte_mb();
> >> +}
> >> +
> >> +static inline void
> >> +virtio_rmb(uint8_t weak_barriers)
> >> +{
> >> +	if (weak_barriers)
> >> +		rte_smp_rmb();
> >> +	else
> >> +		rte_cio_rmb();
> >> +}
> >> +
> >> +static inline void
> >> +virtio_wmb(uint8_t weak_barriers)
> >> +{
> >> +	if (weak_barriers)
> >> +		rte_smp_wmb();
> >> +	else
> >> +		rte_cio_wmb();
> >> +}
> >>  
> >>  #ifdef RTE_PMD_PACKET_PREFETCH
> >>  #define rte_packet_prefetch(p)  rte_prefetch1(p)
> > 
> > 
> > Note kernel uses dma_ barriers not full barriers.
> > I don't know whether it applies to dpdk.
> 
> Yes. I know. I used rte_cio_* barriers which are dpdk equivalent of dma_*
> barriers from kernel.
> 
> > 
> >> @@ -312,7 +337,7 @@ void vq_ring_free_inorder(struct virtqueue *vq, uint16_t desc_idx,
> >>  static inline void
> >>  vq_update_avail_idx(struct virtqueue *vq)
> >>  {
> >> -	virtio_wmb();
> >> +	virtio_wmb(vq->hw->weak_barriers);
> >>  	vq->vq_ring.avail->idx = vq->vq_avail_idx;
> >>  }
> >>  
> >> -- 
> >> 2.17.1
> > 
> > 


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