Re: [PATCH RFC 4/5] vhost-vdpa: support IOTLB batching hints

["Michael S. Tsirkin" <mst@redhat.com>]

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?

> 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?
Or must all changes take place atomically after BATCH_END?
If atomically, we might need to limit batch size to make
sure kernel can keep a copy in memory.


> 
> > 
> > 
> > > 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@redhat.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.

> 
> > 
> > 
> > > 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