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Date:   Mon, 14 Nov 2022 16:11:11 +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: <20221114151111.6ay3ra4shzj3k2lg@sgarzare-redhat>
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On Mon, Nov 14, 2022 at 10:13:51AM +0100, Eugenio Perez Martin wrote:
>On Fri, Nov 11, 2022 at 5:30 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>>
>> 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.
>
>Oh that's a very good point. But then I guess a conformant driver
>should never set more descriptors than that.

Yep, right!

>
>Would it be convenient to make the default queue size of 32768 and let

Yep, I think it makes sense.

>the guest specify less descriptors than that? Default configuration
>will consume more memory then.

Do you mean for the driver point of view?

Because IIUC in vringh we don't allocate anything related to the queue 
size.

>
>> 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.
>>
>
>Maybe it is the right place to do it, but the device is initiated at
>that point so the driver needs to perform a full reset.
>

Yes, but the driver is misbehaving, so it might be okay to request a 
full reset.

>As a reference, qemu will retain the last valid size set to a vq, or
>the default. This is because it ignores the bad values systematically.
>Not sure what is more conformant actually :).
>

Me too :-)

Thanks,
Stefano