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From: Eelco Chaudron <echaudro@redhat.com>
To: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: Gowrishankar Muthukrishnan <gmuthukrishn@marvell.com>,
chenbo.xia@intel.com, dev@dpdk.org
Subject: Re: [EXT] [PATCH] vhost: add device op to offload the interrupt kick
Date: Mon, 03 Apr 2023 16:51:47 +0200
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On 27 Mar 2023, at 18:35, Maxime Coquelin wrote:
> On 3/27/23 18:04, Eelco Chaudron wrote:
>>
>>
>> On 27 Mar 2023, at 17:16, Gowrishankar Muthukrishnan wrote:
>>
>>> Hi Eelco,
>>>
>>>> +void
>>>> +rte_vhost_notify_guest(int vid, uint16_t queue_id) {
>>>> +=09struct virtio_net *dev =3D get_device(vid);
>>>> +=09struct vhost_virtqueue *vq;
>>>> +
>>>> +=09if (!dev || queue_id >=3D VHOST_MAX_VRING)
>>>> +=09=09return;
>>>> +
>>>> +=09vq =3D dev->virtqueue[queue_id];
>>>> +=09if (!vq)
>>>> +=09=09return;
>>>> +
>>>> +=09rte_spinlock_lock(&vq->access_lock);
>>>> +
>>>
>>> Is spin lock needed here before system call ?
>>
>> I assumed access_lock is protecting all the following fields in this str=
ucture, so I need the lock to read the vq->callfd, however, I can/should mo=
ve the eventfd_write outside of the lock.
>
> The FD might be closed between the check and the call to eventfd_write
> though, but I agree this is not optimal to call the eventfd_write under
> the spinlock in your case, as you will block the pmd thread if it tries
> to enqueue/dequeue packets on this queue, defeating the purpose of this
> patch.
>
> Maybe the solution is to change to read-write locks for the access_lock
> spinlock. The datapath (rte_vhost_enqueue_burst/rte_vhost_dequeue_burst)
> and this API would use the read version, meaning they won't lock each
> other, and the control path (lib/vhost/vhost_user.c) will use the write
> version.
>
> Does that make sense?
Hi Maxime, I prepped a patch, but not the read/write part yet, https://gith=
ub.com/chaudron/dpdk/commit/d51c93b4ff08b43daed33e3c0fee193a6d039c25#.
I was thinking that even a read/write lock does not work (or maybe we need =
a combination of taking the read and write lock). As we need to update stat=
istics, which need protection.
For example here you see the split (with just two locks, but the sys call c=
ould be called in the read lock):
https://github.com/chaudron/dpdk/blob/d51c93b4ff08b43daed33e3c0fee193a6d039=
c25/lib/vhost/vhost.c#L1493
The best would be to not have a lock when calling the system call, but that=
does not seem safe. I do not see a clear solution, guess also as I have so=
me trouble understanding the lock rules around vq=E2=80=99s.
Do you have some more insights to share? I can ping you offline and discuss=
this.
Cheers,
Eelco
>>
>>>> +=09if (vq->callfd >=3D 0)
>>>> +=09=09eventfd_write(vq->callfd, (eventfd_t)1);
>>>> +
>>>> +=09rte_spinlock_unlock(&vq->access_lock);
>>>> +}
>>>> +
>>>
>>> Thanks.
>>