[dpdk-dev] [RFC] net/virtio: use real barriers for vDPA
Ilya Maximets
i.maximets at samsung.com
Fri Dec 14 16:56:16 CET 2018
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 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?
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
>>>
>>>
>
>
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