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Date:   Wed, 5 Jul 2023 17:07:11 -0700
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Subject: Re: [PATCH] vdpa: reject F_ENABLE_AFTER_DRIVER_OK if backend does not
 support it
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To:     Eugenio Perez Martin <eperezma@redhat.com>,
        Jason Wang <jasowang@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org
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On 7/5/23 11:27 AM, Eugenio Perez Martin wrote:
 >
> On Wed, Jul 5, 2023 at 9:50 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> On Tue, Jul 4, 2023 at 11:45 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>
>>> On Tue, Jul 04, 2023 at 01:36:11PM +0200, Eugenio Perez Martin wrote:
>>>> On Tue, Jul 4, 2023 at 12:38 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>>>
>>>>> On Tue, Jul 04, 2023 at 12:25:32PM +0200, Eugenio Perez Martin wrote:
>>>>>> On Mon, Jul 3, 2023 at 4:52 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>>>>>
>>>>>>> On Mon, Jul 03, 2023 at 04:22:18PM +0200, Eugenio Pérez wrote:
>>>>>>>> With the current code it is accepted as long as userland send it.
>>>>>>>>
>>>>>>>> Although userland should not set a feature flag that has not been
>>>>>>>> offered to it with VHOST_GET_BACKEND_FEATURES, the current code will not
>>>>>>>> complain for it.
>>>>>>>>
>>>>>>>> Since there is no specific reason for any parent to reject that backend
>>>>>>>> feature bit when it has been proposed, let's control it at vdpa frontend
>>>>>>>> level. Future patches may move this control to the parent driver.
>>>>>>>>
>>>>>>>> Fixes: 967800d2d52e ("vdpa: accept VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK backend feature")
>>>>>>>> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
>>>>>>>
>>>>>>> Please do send v3. And again, I don't want to send "after driver ok" hack
>>>>>>> upstream at all, I merged it in next just to give it some testing.
>>>>>>> We want RING_ACCESS_AFTER_KICK or some such.
>>>>>>>
>>>>>>
>>>>>> Current devices do not support that semantic.
>>>>>
>>>>> Which devices specifically access the ring after DRIVER_OK but before
>>>>> a kick?

The PDS vdpa device can deal with a call to .set_vq_ready after 
DRIVER_OK is set.  And I'm told that our VQ activity should start 
without a kick.

Our vdpa device FW doesn't currently have support for 
VIRTIO_F_RING_RESET, but I believe it could be added without too much 
trouble.

sln


>>>>>
>>>>
>>>> Previous versions of the QEMU LM series did a spurious kick to start
>>>> traffic at the LM destination [1]. When it was proposed, that spurious
>>>> kick was removed from the series because to check for descriptors
>>>> after driver_ok, even without a kick, was considered work of the
>>>> parent driver.
>>>>
>>>> I'm ok to go back to this spurious kick, but I'm not sure if the hw
>>>> will read the ring before the kick actually. I can ask.
>>>>
>>>> Thanks!
>>>>
>>>> [1] https://lists.nongnu.org/archive/html/qemu-devel/2023-01/msg02775.html
>>>
>>> Let's find out. We need to check for ENABLE_AFTER_DRIVER_OK too, no?
>>
>> My understanding is [1] assuming ACCESS_AFTER_KICK. This seems
>> sub-optimal than assuming ENABLE_AFTER_DRIVER_OK.
>>
>> But this reminds me one thing, as the thread is going too long, I
>> wonder if we simply assume ENABLE_AFTER_DRIVER_OK if RING_RESET is
>> supported?
>>
> 
> The problem with that is that the device needs to support all
> RING_RESET, like to be able to change vq address etc after DRIVER_OK.
> Not all HW support it.
> 
> We just need the subset of having the dataplane freezed until all CVQ
> commands have been consumed. I'm sure current vDPA code already
> supports it in some devices, like MLX and PSD.
> 
> Thanks!
> 
>> Thanks
>>
>>>
>>>
>>>
>>>>>> My plan was to convert
>>>>>> it in vp_vdpa if needed, and reuse the current vdpa ops. Sorry if I
>>>>>> was not explicit enough.
>>>>>>
>>>>>> The only solution I can see to that is to trap & emulate in the vdpa
>>>>>> (parent?) driver, as talked in virtio-comment. But that complicates
>>>>>> the architecture:
>>>>>> * Offer VHOST_BACKEND_F_RING_ACCESS_AFTER_KICK
>>>>>> * Store vq enable state separately, at
>>>>>> vdpa->config->set_vq_ready(true), but not transmit that enable to hw
>>>>>> * Store the doorbell state separately, but do not configure it to the
>>>>>> device directly.
>>>>>>
>>>>>> But how to recover if the device cannot configure them at kick time,
>>>>>> for example?
>>>>>>
>>>>>> Maybe we can just fail if the parent driver does not support enabling
>>>>>> the vq after DRIVER_OK? That way no new feature flag is needed.
>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>>>
>>>>>>>> ---
>>>>>>>> Sent with Fixes: tag pointing to git.kernel.org/pub/scm/linux/kernel/git/mst
>>>>>>>> commit. Please let me know if I should send a v3 of [1] instead.
>>>>>>>>
>>>>>>>> [1] https://lore.kernel.org/lkml/20230609121244-mutt-send-email-mst@kernel.org/T/
>>>>>>>> ---
>>>>>>>>   drivers/vhost/vdpa.c | 7 +++++--
>>>>>>>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
>>>>>>>> index e1abf29fed5b..a7e554352351 100644
>>>>>>>> --- a/drivers/vhost/vdpa.c
>>>>>>>> +++ b/drivers/vhost/vdpa.c
>>>>>>>> @@ -681,18 +681,21 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
>>>>>>>>   {
>>>>>>>>        struct vhost_vdpa *v = filep->private_data;
>>>>>>>>        struct vhost_dev *d = &v->vdev;
>>>>>>>> +     const struct vdpa_config_ops *ops = v->vdpa->config;
>>>>>>>>        void __user *argp = (void __user *)arg;
>>>>>>>>        u64 __user *featurep = argp;
>>>>>>>> -     u64 features;
>>>>>>>> +     u64 features, parent_features = 0;
>>>>>>>>        long r = 0;
>>>>>>>>
>>>>>>>>        if (cmd == VHOST_SET_BACKEND_FEATURES) {
>>>>>>>>                if (copy_from_user(&features, featurep, sizeof(features)))
>>>>>>>>                        return -EFAULT;
>>>>>>>> +             if (ops->get_backend_features)
>>>>>>>> +                     parent_features = ops->get_backend_features(v->vdpa);
>>>>>>>>                if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
>>>>>>>>                                 BIT_ULL(VHOST_BACKEND_F_SUSPEND) |
>>>>>>>>                                 BIT_ULL(VHOST_BACKEND_F_RESUME) |
>>>>>>>> -                              BIT_ULL(VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK)))
>>>>>>>> +                              parent_features))
>>>>>>>>                        return -EOPNOTSUPP;
>>>>>>>>                if ((features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) &&
>>>>>>>>                     !vhost_vdpa_can_suspend(v))
>>>>>>>> --
>>>>>>>> 2.39.3
>>>>>>>
>>>>>
>>>
>>
>