Re: [RFC] net/virtio: use real barriers for vDPA

[Ilya Maximets <i.maximets@samsung.com>]

On 14.12.2018 18:01, Michael S. Tsirkin wrote:
> 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@dpdk.org
>>>>
>>>> Signed-off-by: Ilya Maximets <i.maximets@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?

But this hack allows not to change the QEMU.

> 
>> 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.

OK.

> 
>> 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.

OK. So, I prepared the patch that actually implements the VIRTIO_F_ORDER_PLATFORM:
    http://mails.dpdk.org/archives/dev/2018-December/121031.html
And this should be enough from the DPDK side in case vDPA HW offers the feature.

> Exposing a new feature bit is not a lot of work at all.

It depends. However this one should be simple.
I'll prepare patch for QEMU.

> 
>> 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.

OK.

> 
> 
>>>
>>>
>>>> ---
>>>>
>>>> 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
>>>
>>>
> 
>