[v1] vhost: support cross page buf in async data path
diff mbox series

Message ID 20200720025242.3066787-1-patrick.fu@intel.com
State Superseded, archived
Delegated to: Maxime Coquelin
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  • [v1] vhost: support cross page buf in async data path
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Commit Message

Patrick Fu July 20, 2020, 2:52 a.m. UTC
From: Patrick Fu <patrick.fu@intel.com>

Async copy fails when ring buffer cross two physical pages. This patch
fix the failure by letting copies occur in sync mode if crossing page
buffers are given.

Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")

Signed-off-by: Patrick Fu <patrick.fu@intel.com>
---
 lib/librte_vhost/virtio_net.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

Comments

Maxime Coquelin July 20, 2020, 4:39 p.m. UTC | #1
The title could be improved, it is not very clear IMHO.

On 7/20/20 4:52 AM, patrick.fu@intel.com wrote:
> From: Patrick Fu <patrick.fu@intel.com>
> 
> Async copy fails when ring buffer cross two physical pages. This patch
> fix the failure by letting copies occur in sync mode if crossing page
> buffers are given.

Wouldn't it be possible to have the buffer split into two iovecs?

> Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")
> 
> Signed-off-by: Patrick Fu <patrick.fu@intel.com>
> ---
>  lib/librte_vhost/virtio_net.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index 1d0be3dd4..44b22a8ad 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -1071,16 +1071,10 @@ async_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  		}
>  
>  		cpy_len = RTE_MIN(buf_avail, mbuf_avail);
> +		hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
> +				buf_iova + buf_offset, cpy_len);
>  
> -		if (unlikely(cpy_len >= cpy_threshold)) {
> -			hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
> -					buf_iova + buf_offset, cpy_len);
> -
> -			if (unlikely(!hpa)) {
> -				error = -1;
> -				goto out;
> -			}
> -
> +		if (unlikely(cpy_len >= cpy_threshold && hpa)) {
>  			async_fill_vec(src_iovec + tvec_idx,
>  				(void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
>  						mbuf_offset), cpy_len);
>
Patrick Fu July 21, 2020, 2:57 a.m. UTC | #2
Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Tuesday, July 21, 2020 12:40 AM
> To: Fu, Patrick <patrick.fu@intel.com>; dev@dpdk.org; Xia, Chenbo
> <chenbo.xia@intel.com>
> Subject: Re: [PATCH v1] vhost: support cross page buf in async data path
> 
> The title could be improved, it is not very clear IMHO.
How about: 
vhost: fix async copy failure on buffers cross page boundary

> On 7/20/20 4:52 AM, patrick.fu@intel.com wrote:
> > From: Patrick Fu <patrick.fu@intel.com>
> >
> > Async copy fails when ring buffer cross two physical pages. This patch
> > fix the failure by letting copies occur in sync mode if crossing page
> > buffers are given.
> 
> Wouldn't it be possible to have the buffer split into two iovecs?
Technically we can do that, however, it will also introduce significant overhead:
 - overhead from adding additional logic in vhost async data path to handle the case
 - overhead from dma device to consume 2 iovecs
In average, I don't think dma copy can benefit too much for the buffer which are split into multiple pages. 
CPU copy shall be a more suitable method.

 
> > Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")
> >
> > Signed-off-by: Patrick Fu <patrick.fu@intel.com>
> > ---
> >  lib/librte_vhost/virtio_net.c | 12 +++---------
> >  1 file changed, 3 insertions(+), 9 deletions(-)
> >
> > diff --git a/lib/librte_vhost/virtio_net.c
> > b/lib/librte_vhost/virtio_net.c index 1d0be3dd4..44b22a8ad 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -1071,16 +1071,10 @@ async_mbuf_to_desc(struct virtio_net *dev,
> struct vhost_virtqueue *vq,
> >  		}
> >
> >  		cpy_len = RTE_MIN(buf_avail, mbuf_avail);
> > +		hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
> > +				buf_iova + buf_offset, cpy_len);
> >
> > -		if (unlikely(cpy_len >= cpy_threshold)) {
> > -			hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
> > -					buf_iova + buf_offset, cpy_len);
> > -
> > -			if (unlikely(!hpa)) {
> > -				error = -1;
> > -				goto out;
> > -			}
> > -
> > +		if (unlikely(cpy_len >= cpy_threshold && hpa)) {
> >  			async_fill_vec(src_iovec + tvec_idx,
> >  				(void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
> >  						mbuf_offset), cpy_len);
> >
Maxime Coquelin July 21, 2020, 8:35 a.m. UTC | #3
Hi Patrick,

On 7/21/20 4:57 AM, Fu, Patrick wrote:
> Hi Maxime,
> 
>> -----Original Message-----
>> From: Maxime Coquelin <maxime.coquelin@redhat.com>
>> Sent: Tuesday, July 21, 2020 12:40 AM
>> To: Fu, Patrick <patrick.fu@intel.com>; dev@dpdk.org; Xia, Chenbo
>> <chenbo.xia@intel.com>
>> Subject: Re: [PATCH v1] vhost: support cross page buf in async data path
>>
>> The title could be improved, it is not very clear IMHO.
> How about: 
> vhost: fix async copy failure on buffers cross page boundary
> 
>> On 7/20/20 4:52 AM, patrick.fu@intel.com wrote:
>>> From: Patrick Fu <patrick.fu@intel.com>
>>>
>>> Async copy fails when ring buffer cross two physical pages. This patch
>>> fix the failure by letting copies occur in sync mode if crossing page
>>> buffers are given.
>>
>> Wouldn't it be possible to have the buffer split into two iovecs?
> Technically we can do that, however, it will also introduce significant overhead:
>  - overhead from adding additional logic in vhost async data path to handle the case
>  - overhead from dma device to consume 2 iovecs
> In average, I don't think dma copy can benefit too much for the buffer which are split into multiple pages. 
> CPU copy shall be a more suitable method.

I think we should try, that would make a cleaner implementation. I don't
think having to fallback to sync mode is a good idea because it adds an
overhead on the CPU, which is what we try to avoid with this async mode.

Also, I am not convinced the overhead would be that significant, at
least I hope so, otherwise it would mean this new path is just
performing better because it takes a lot of shortcuts, like the vector
path in Virtio PMD.

Regards,
Maxime

>  
>>> Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")
>>>
>>> Signed-off-by: Patrick Fu <patrick.fu@intel.com>
>>> ---
>>>  lib/librte_vhost/virtio_net.c | 12 +++---------
>>>  1 file changed, 3 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/lib/librte_vhost/virtio_net.c
>>> b/lib/librte_vhost/virtio_net.c index 1d0be3dd4..44b22a8ad 100644
>>> --- a/lib/librte_vhost/virtio_net.c
>>> +++ b/lib/librte_vhost/virtio_net.c
>>> @@ -1071,16 +1071,10 @@ async_mbuf_to_desc(struct virtio_net *dev,
>> struct vhost_virtqueue *vq,
>>>  		}
>>>
>>>  		cpy_len = RTE_MIN(buf_avail, mbuf_avail);
>>> +		hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
>>> +				buf_iova + buf_offset, cpy_len);
>>>
>>> -		if (unlikely(cpy_len >= cpy_threshold)) {
>>> -			hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
>>> -					buf_iova + buf_offset, cpy_len);
>>> -
>>> -			if (unlikely(!hpa)) {
>>> -				error = -1;
>>> -				goto out;
>>> -			}
>>> -
>>> +		if (unlikely(cpy_len >= cpy_threshold && hpa)) {
>>>  			async_fill_vec(src_iovec + tvec_idx,
>>>  				(void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
>>>  						mbuf_offset), cpy_len);
>>>
>
Patrick Fu July 21, 2020, 9:01 a.m. UTC | #4
Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Tuesday, July 21, 2020 4:35 PM
> To: Fu, Patrick <patrick.fu@intel.com>; dev@dpdk.org; Xia, Chenbo
> <chenbo.xia@intel.com>
> Subject: Re: [PATCH v1] vhost: support cross page buf in async data path
> 
> Hi Patrick,
> 
> On 7/21/20 4:57 AM, Fu, Patrick wrote:
> > Hi Maxime,
> >
> >> -----Original Message-----
> >> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> >> Sent: Tuesday, July 21, 2020 12:40 AM
> >> To: Fu, Patrick <patrick.fu@intel.com>; dev@dpdk.org; Xia, Chenbo
> >> <chenbo.xia@intel.com>
> >> Subject: Re: [PATCH v1] vhost: support cross page buf in async data
> >> path
> >>
> >> The title could be improved, it is not very clear IMHO.
> > How about:
> > vhost: fix async copy failure on buffers cross page boundary
> >
> >> On 7/20/20 4:52 AM, patrick.fu@intel.com wrote:
> >>> From: Patrick Fu <patrick.fu@intel.com>
> >>>
> >>> Async copy fails when ring buffer cross two physical pages. This
> >>> patch fix the failure by letting copies occur in sync mode if
> >>> crossing page buffers are given.
> >>
> >> Wouldn't it be possible to have the buffer split into two iovecs?
> > Technically we can do that, however, it will also introduce significant
> overhead:
> >  - overhead from adding additional logic in vhost async data path to
> > handle the case
> >  - overhead from dma device to consume 2 iovecs In average, I don't
> > think dma copy can benefit too much for the buffer which are split into
> multiple pages.
> > CPU copy shall be a more suitable method.
> 
> I think we should try, that would make a cleaner implementation. I don't
> think having to fallback to sync mode is a good idea because it adds an
> overhead on the CPU, which is what we try to avoid with this async mode.
> 
> Also, I am not convinced the overhead would be that significant, at least I
> hope so, otherwise it would mean this new path is just performing better
> because it takes a lot of shortcuts, like the vector path in Virtio PMD.

I can make a trial patch and do some comparison. I will try to feedback the result by this weekend.

Thanks,

Patrick

Patch
diff mbox series

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 1d0be3dd4..44b22a8ad 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -1071,16 +1071,10 @@  async_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		}
 
 		cpy_len = RTE_MIN(buf_avail, mbuf_avail);
+		hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
+				buf_iova + buf_offset, cpy_len);
 
-		if (unlikely(cpy_len >= cpy_threshold)) {
-			hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
-					buf_iova + buf_offset, cpy_len);
-
-			if (unlikely(!hpa)) {
-				error = -1;
-				goto out;
-			}
-
+		if (unlikely(cpy_len >= cpy_threshold && hpa)) {
 			async_fill_vec(src_iovec + tvec_idx,
 				(void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
 						mbuf_offset), cpy_len);