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Date: Tue, 30 Jun 2020 09:55:23 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, rob.miller@...adcom.com,
lingshan.zhu@...el.com, eperezma@...hat.com, lulu@...hat.com,
shahafs@...lanox.com, hanand@...inx.com, mhabets@...arflare.com,
gdawar@...inx.com, saugatm@...inx.com, vmireyno@...vell.com,
zhangweining@...jie.com.cn, eli@...lanox.com
Subject: Re: [PATCH RFC 4/5] vhost-vdpa: support IOTLB batching hints
On 2020/6/29 下午11:49, Michael S. Tsirkin wrote:
> On Mon, Jun 29, 2020 at 05:26:03PM +0800, Jason Wang wrote:
>> On 2020/6/28 下午5:58, Michael S. Tsirkin wrote:
>>> On Thu, Jun 18, 2020 at 01:56:25PM +0800, Jason Wang wrote:
>>>> This patches extend the vhost IOTLB API to accept batch updating hints
>>>> form userspace. When userspace wants update the device IOTLB in a
>>>> batch, it may do:
>>>>
>>>> 1) Write vhost_iotlb_msg with VHOST_IOTLB_BATCH_BEGIN flag
>>>> 2) Perform a batch of IOTLB updating via VHOST_IOTLB_UPDATE/INVALIDATE
>>>> 3) Write vhost_iotlb_msg with VHOST_IOTLB_BATCH_END flag
>>> As long as we are extending the interface,
>>> is there some way we could cut down the number of system calls needed
>>> here?
>>
>> I'm not sure it's worth to do that since usually we only have less than 10
>> regions.
>
> Well with a guest iommu I'm guessing it can go up significantly, right?
The batching flag is not mandatory, so userspace can still choose to
update a single IOTLB mapping through a single system call without batch
flag.
>
>> A possible method is to carry multiple vhost_iotlb_message in one system
>> call.
> That's an option.
> Also, can kernel handle a batch that is too large by applying
> parts of it iteratively?
I think so, we can iterate a vhost iotlb message one by one through iov
iterator.
> Or must all changes take place atomically after BATCH_END?
For changes:
- if you mean the changes in vhost IOTLB, it's not atomically
- if you mean the mapping seen by vDPA device driver, it could be
atomically for the device driver that implements set_map() method, since
we pass a interval tree to the device.
> If atomically, we might need to limit batch size to make
> sure kernel can keep a copy in memory.
It's the responsibility of guest driver to make sure there won't be a
race condition between DMA and map updating. So it looks to me we don't
need to care about that.
>
>
>>>
>>>> Vhost-vdpa may decide to batch the IOMMU/IOTLB updating in step 3 when
>>>> vDPA device support set_map() ops. This is useful for the vDPA device
>>>> that want to know all the mappings to tweak their own DMA translation
>>>> logic.
>>>>
>>>> For vDPA device that doesn't require set_map(), no behavior changes.
>>>>
>>>> This capability is advertised via VHOST_BACKEND_F_IOTLB_BATCH capability.
>>>>
>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>>> ---
>>>> drivers/vhost/vdpa.c | 30 +++++++++++++++++++++++-------
>>>> include/uapi/linux/vhost.h | 2 ++
>>>> include/uapi/linux/vhost_types.h | 7 +++++++
>>>> 3 files changed, 32 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
>>>> index 453057421f80..8f624bbafee7 100644
>>>> --- a/drivers/vhost/vdpa.c
>>>> +++ b/drivers/vhost/vdpa.c
>>>> @@ -56,7 +56,9 @@ enum {
>>>> };
>>>> enum {
>>>> - VHOST_VDPA_BACKEND_FEATURES = (1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2)
>>>> + VHOST_VDPA_BACKEND_FEATURES =
>>>> + (1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2) |
>>>> + (1ULL << VHOST_BACKEND_F_IOTLB_BATCH),
>>>> };
>>>> /* Currently, only network backend w/o multiqueue is supported. */
>>>> @@ -77,6 +79,7 @@ struct vhost_vdpa {
>>>> int virtio_id;
>>>> int minor;
>>>> struct eventfd_ctx *config_ctx;
>>>> + int in_batch;
>>>> };
>>>> static DEFINE_IDA(vhost_vdpa_ida);
>>>> @@ -125,6 +128,7 @@ static void vhost_vdpa_reset(struct vhost_vdpa *v)
>>>> const struct vdpa_config_ops *ops = vdpa->config;
>>>> ops->set_status(vdpa, 0);
>>>> + v->in_batch = 0;
>>>> }
>>>> static long vhost_vdpa_get_device_id(struct vhost_vdpa *v, u8 __user *argp)
>>>> @@ -540,9 +544,10 @@ static int vhost_vdpa_map(struct vhost_vdpa *v,
>>>> if (ops->dma_map)
>>>> r = ops->dma_map(vdpa, iova, size, pa, perm);
>>>> - else if (ops->set_map)
>>>> - r = ops->set_map(vdpa, dev->iotlb);
>>>> - else
>>>> + else if (ops->set_map) {
>>>> + if (!v->in_batch)
>>>> + r = ops->set_map(vdpa, dev->iotlb);
>>>> + } else
>>>> r = iommu_map(v->domain, iova, pa, size,
>>>> perm_to_iommu_flags(perm));
>>>> @@ -559,9 +564,10 @@ static void vhost_vdpa_unmap(struct vhost_vdpa *v, u64 iova, u64 size)
>>>> if (ops->dma_map)
>>>> ops->dma_unmap(vdpa, iova, size);
>>>> - else if (ops->set_map)
>>>> - ops->set_map(vdpa, dev->iotlb);
>>>> - else
>>>> + else if (ops->set_map) {
>>>> + if (!v->in_batch)
>>>> + ops->set_map(vdpa, dev->iotlb);
>>>> + } else
>>>> iommu_unmap(v->domain, iova, size);
>>>> }
>>>> @@ -655,6 +661,8 @@ static int vhost_vdpa_process_iotlb_msg(struct vhost_dev *dev,
>>>> struct vhost_iotlb_msg *msg)
>>>> {
>>>> struct vhost_vdpa *v = container_of(dev, struct vhost_vdpa, vdev);
>>>> + struct vdpa_device *vdpa = v->vdpa;
>>>> + const struct vdpa_config_ops *ops = vdpa->config;
>>>> int r = 0;
>>>> r = vhost_dev_check_owner(dev);
>>>> @@ -668,6 +676,14 @@ static int vhost_vdpa_process_iotlb_msg(struct vhost_dev *dev,
>>>> case VHOST_IOTLB_INVALIDATE:
>>>> vhost_vdpa_unmap(v, msg->iova, msg->size);
>>>> break;
>>>> + case VHOST_IOTLB_BATCH_BEGIN:
>>>> + v->in_batch = true;
>>>> + break;
>>>> + case VHOST_IOTLB_BATCH_END:
>>>> + if (v->in_batch && ops->set_map)
>>>> + ops->set_map(vdpa, dev->iotlb);
>>>> + v->in_batch = false;
>>>> + break;
>>>> default:
>>>> r = -EINVAL;
>>>> break;
>>>> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
>>>> index 0c2349612e77..565da96f55d5 100644
>>>> --- a/include/uapi/linux/vhost.h
>>>> +++ b/include/uapi/linux/vhost.h
>>>> @@ -91,6 +91,8 @@
>>>> /* Use message type V2 */
>>>> #define VHOST_BACKEND_F_IOTLB_MSG_V2 0x1
>>>> +/* IOTLB can accpet batching hints */
>>> typo
>>
>> Will fix.
>>
>>
>>>> +#define VHOST_BACKEND_F_IOTLB_BATCH 0x2
>>>> #define VHOST_SET_BACKEND_FEATURES _IOW(VHOST_VIRTIO, 0x25, __u64)
>>>> #define VHOST_GET_BACKEND_FEATURES _IOR(VHOST_VIRTIO, 0x26, __u64)
>>>> diff --git a/include/uapi/linux/vhost_types.h b/include/uapi/linux/vhost_types.h
>>>> index 669457ce5c48..5c12faffdde9 100644
>>>> --- a/include/uapi/linux/vhost_types.h
>>>> +++ b/include/uapi/linux/vhost_types.h
>>>> @@ -60,6 +60,13 @@ struct vhost_iotlb_msg {
>>>> #define VHOST_IOTLB_UPDATE 2
>>>> #define VHOST_IOTLB_INVALIDATE 3
>>>> #define VHOST_IOTLB_ACCESS_FAIL 4
>>>> +/* VHOST_IOTLB_BATCH_BEGIN is a hint that userspace will update
>>>> + * several mappings afterwards. VHOST_IOTLB_BATCH_END is a hint that
>>>> + * userspace had finished the mapping updating.
>>> Well not just hints - in fact updates do not take place
>>> until _END.
>>>
>>> How about:
>>>
>>> /* VHOST_IOTLB_BATCH_BEGIN and VHOST_IOTLB_BATCH_END allow modifying
>>> * multiple mappings in one go: beginning with
>>> * VHOST_IOTLB_BATCH_BEGIN, followed by any number of
>>> VHOST_IOTLB_UPDATE messages, and ending with VHOST_IOTLB_BATCH_END.
>>> */
>>
>> That's better.
>
> Is there a guarantee that these changes take place atomically?
> Let's document that.
There's no guarantee. Will document this.
Thanks
>
>>>
>>>> When those two flags
>>>> + * were set, kernel will ignore the rest fileds of the IOTLB message.
>>> how about:
>>>
>>> when one of these two values is used as the message type, the
>>> rest of the fields in the message are ignored.
>>
>> Yes.
>>
>> Will fix.
>>
>> Thanks
>>
>>
>>>> + */
>>>> +#define VHOST_IOTLB_BATCH_BEGIN 5
>>>> +#define VHOST_IOTLB_BATCH_END 6
>>>> __u8 type;
>>>> };
>>>> --
>>>> 2.20.1
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