linux-kernel - Re: [RFC] virtio: support VIRTIO_F_IO

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Message-ID: <efe1b3b9-3601-d861-aff5-80d10c9f0cdb@redhat.com>
Date:   Thu, 3 May 2018 20:57:33 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>
Cc:     mst@...hat.com, pbonzini@...hat.com, stefanha@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, dan.daly@...el.com,
        cunming.liang@...el.com, zhihong.wang@...el.com
Subject: Re: [RFC] virtio: support VIRTIO_F_IO_BARRIER



On 2018年05月03日 17:58, Tiwei Bie wrote:
> On Thu, May 03, 2018 at 05:09:44PM +0800, Jason Wang wrote:
>> On 2018年05月03日 16:30, Tiwei Bie wrote:
>>> On Thu, May 03, 2018 at 03:30:03PM +0800, Jason Wang wrote:
>>>> On 2018年05月03日 10:59, Tiwei Bie wrote:
>>>>> This patch introduces the support for VIRTIO_F_IO_BARRIER.
>>>>> When this feature is negotiated, driver will use the barriers
>>>>> suitable for hardware devices.
>>>>>
>>>>> Signed-off-by: Tiwei Bie <tiwei.bie@...el.com>
>>>>> ---
>>>>>     drivers/virtio/virtio_ring.c       | 5 +++++
>>>>>     include/uapi/linux/virtio_config.h | 8 +++++++-
>>>>>     2 files changed, 12 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>>>>> index 21d464a29cf8..edb565643bf4 100644
>>>>> --- a/drivers/virtio/virtio_ring.c
>>>>> +++ b/drivers/virtio/virtio_ring.c
>>>>> @@ -996,6 +996,9 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>>>>>     		!context;
>>>>>     	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
>>>>> +	if (virtio_has_feature(vdev, VIRTIO_F_IO_BARRIER))
>>>>> +		vq->weak_barriers = false;
>>>>> +
>>>>>     	/* No callback?  Tell other side not to bother us. */
>>>>>     	if (!callback) {
>>>>>     		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
>>>>> @@ -1164,6 +1167,8 @@ void vring_transport_features(struct virtio_device *vdev)
>>>>>     			break;
>>>>>     		case VIRTIO_F_IOMMU_PLATFORM:
>>>>>     			break;
>>>>> +		case VIRTIO_F_IO_BARRIER:
>>>>> +			break;
>>>>>     		default:
>>>>>     			/* We don't understand this bit. */
>>>>>     			__virtio_clear_bit(vdev, i);
>>>>> diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
>>>>> index 308e2096291f..6ca8d24bf468 100644
>>>>> --- a/include/uapi/linux/virtio_config.h
>>>>> +++ b/include/uapi/linux/virtio_config.h
>>>>> @@ -49,7 +49,7 @@
>>>>>      * transport being used (eg. virtio_ring), the rest are per-device feature
>>>>>      * bits. */
>>>>>     #define VIRTIO_TRANSPORT_F_START	28
>>>>> -#define VIRTIO_TRANSPORT_F_END		34
>>>>> +#define VIRTIO_TRANSPORT_F_END		38
>>>>>     #ifndef VIRTIO_CONFIG_NO_LEGACY
>>>>>     /* Do we get callbacks when the ring is completely used, even if we've
>>>>> @@ -71,4 +71,10 @@
>>>>>      * this is for compatibility with legacy systems.
>>>>>      */
>>>>>     #define VIRTIO_F_IOMMU_PLATFORM		33
>>>>> +
>>>>> +/*
>>>>> + * If clear - driver may use barriers suitable for CPU cores.
>>>>> + * If set - driver must use barriers suitable for hardware devices.
>>>>> + */
>>>>> +#define VIRTIO_F_IO_BARRIER		37
>>>>>     #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */
>>>> Hi:
>>>>
>>>> I believe this depends on Michael's patch of
>>>>
>>>> "[PATCH] virtio_ring: switch to dma_XX barriers for rpmsg"
>>>>
>>>> ?
>>>>
>>>> Thanks
>>> We already have below commit and some other related commits
>>> in the tree:
>>>
>>> 7b21e34fd1c2 ("virtio: harsher barriers for rpmsg.")
>>>
>>> They should have already guaranteed that virtio_Xmb() will
>>> be OK for hardware devices when vq->weak_barriers is false.
>>> If my understanding is correct, the barriers used in this
>>> case are overkill. So Michael's patch is to make the barriers
>>> weaker (or better).
>>>
>>> Best regards,
>>> Tiwei Bie
>> Well, I think we need dma barriers for some platforms according to previous
>> discussion? Without Michael's patch, we won't use any dma barriers in fact
>> for virtio.
> You are right. Thanks! Are you suggesting to add a
> reference to Michael's patch in this patch to make
> sure that it won't be applied before that patch?

Yes, better to have some notes, or maybe just squash that patch into 
this series.

Let's wait for Michael's reply to see.

Thanks

>
> Best regards,
> Tiwei Bie
>
>> Thanks