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Date:   Fri, 11 Nov 2022 17:30:07 +0100
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Eugenio Perez Martin <eperezma@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vdpa_sim: fix vringh initialization in
 vdpasim_queue_ready()
Message-ID: <20221111163007.35kvkodvk6zpimmu@sgarzare-redhat>
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On Fri, Nov 11, 2022 at 04:40:33PM +0100, Eugenio Perez Martin wrote:
>On Thu, Nov 10, 2022 at 3:13 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>>
>> When we initialize vringh, we should pass the features and the
>> number of elements in the virtqueue negotiated with the driver,
>> otherwise operations with vringh may fail.
>>
>> This was discovered in a case where the driver sets a number of
>> elements in the virtqueue different from the value returned by
>> .get_vq_num_max().
>>
>> In vdpasim_vq_reset() is safe to initialize the vringh with
>> default values, since the virtqueue will not be used until
>> vdpasim_queue_ready() is called again.
>>
>> Fixes: 2c53d0f64c06 ("vdpasim: vDPA device simulator")
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>>  drivers/vdpa/vdpa_sim/vdpa_sim.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
>> index b071f0d842fb..b20689f8fe89 100644
>> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
>> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
>> @@ -67,8 +67,7 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
>>  {
>>         struct vdpasim_virtqueue *vq = &vdpasim->vqs[idx];
>>
>> -       vringh_init_iotlb(&vq->vring, vdpasim->dev_attr.supported_features,
>> -                         VDPASIM_QUEUE_MAX, false,
>> +       vringh_init_iotlb(&vq->vring, vdpasim->features, vq->num, false,
>>                           (struct vring_desc *)(uintptr_t)vq->desc_addr,
>>                           (struct vring_avail *)
>>                           (uintptr_t)vq->driver_addr,
>> --
>> 2.38.1
>>
>
>I think this is definitely an improvement, but I'd say we should go a
>step further and rename VDPASIM_QUEUE_MAX to VDPASIM_QUEUE_DEFAULT. As
>you point out in the patch message it is not a max anymore.

I'm not sure about renaming since it is the value returned by 
vdpasim_get_vq_num_max, so IMHO the _MAX suffix is fine.
But I admit that initially I didn't understand whether it's the maximum 
number of queues or elements, so maybe VDPASIM_VQ_NUM_MAX is better.

>
>Another thing to note is that we don't have a way to report that
>userspace indicated a bad value for queue length. With the current
>code vringh will not initialize at all if I'm not wrong, so we should
>prevent userspace to put a bad num.

Right!

>
>Ideally, we should repeat the tests of vring_init_kern at
>vdpasim_set_vq_num. We could either call it with NULL vring addresses
>to check for -EINVAL, or simply repeat the conditional (!num || num >
>0xffff || (num & (num - 1))). I'd say the first one is better to not
>go out of sync.

Or we could do the check in vdpasim_set_vq_ready() and set it not ready 
if the vq_num is wrong.

>
>All of that can be done on top anyway, so for this patch:
>
>Acked-by: Eugenio P?rez <eperezma@redhat.com>
>

Thanks for the review,
Stefano