linux-kernel - Re: [PATCH] vdpa: conditionally fill max max queue pair for stats

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Date:   Wed, 7 Sep 2022 14:53:19 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eli Cohen <elic@...dia.com>
Cc:     "mst@...hat.com" <mst@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vdpa: conditionally fill max max queue pair for stats

On Wed, Sep 7, 2022 at 2:11 PM Eli Cohen <elic@...dia.com> wrote:
>
> > From: Jason Wang <jasowang@...hat.com>
> > Sent: Wednesday, 7 September 2022 9:01
> > To: mst@...hat.com; jasowang@...hat.com; Eli Cohen <elic@...dia.com>;
> > virtualization@...ts.linux-foundation.org; linux-kernel@...r.kernel.org
> > Subject: [PATCH] vdpa: conditionally fill max max queue pair for stats
> >
> > For the device without multiqueue feature, we will read 0 as
> > max_virtqueue_pairs from the config.
> If this is the case for other vdpa vendor drivers, shouldn't we fix it there? After all,
> config->max_virtqueue_pairs should always show valid values.

Not for the case when the device doesn't offer MQ. According to the
spec, the max_virtqueue_pairs doesn't exist in this case.

>
> > So if we fill
> > VDPA_ATTR_DEV_NET_CFG_MAX_VQP with the value we read from the
> > config
> > we will confuse the user.
> >
> > Fixing this by only filling the value when multiqueue is offered by
> > the device so userspace can assume 1 when the attr is not provided.
> >
> > Fixes: 13b00b135665c("vdpa: Add support for querying vendor statistics")
> > Cc: Eli Cohen <elic@...dia.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > ---
> >  drivers/vdpa/vdpa.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> > index c06c02704461..bc328197263f 100644
> > --- a/drivers/vdpa/vdpa.c
> > +++ b/drivers/vdpa/vdpa.c
> > @@ -894,7 +894,6 @@ static int vdpa_fill_stats_rec(struct vdpa_device
> > *vdev, struct sk_buff *msg,
> >  {
> >       struct virtio_net_config config = {};
> >       u64 features;
> > -     u16 max_vqp;
> >       u8 status;
> >       int err;
> >
> > @@ -905,15 +904,15 @@ static int vdpa_fill_stats_rec(struct vdpa_device
> > *vdev, struct sk_buff *msg,
> >       }
> >       vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
> >
> > -     max_vqp = __virtio16_to_cpu(true, config.max_virtqueue_pairs);
> > -     if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MAX_VQP,
> > max_vqp))
> > -             return -EMSGSIZE;
> > -
> >       features = vdev->config->get_driver_features(vdev);
> >       if (nla_put_u64_64bit(msg,
> > VDPA_ATTR_DEV_NEGOTIATED_FEATURES,
> >                             features, VDPA_ATTR_PAD))
> >               return -EMSGSIZE;
> >
> > +     err = vdpa_dev_net_mq_config_fill(vdev, msg, features, &config);
> > +     if (err)
> > +             return err;
> > +
>
> So that means that you can't read statistics when MQ is not supported. Is this worth sacrificing?

vdpa_dev_net_mq_config_fill() will return 0 in the case of !MQ, so it
should still work.

Thanks


>
> >       if (nla_put_u32(msg, VDPA_ATTR_DEV_QUEUE_INDEX, index))
> >               return -EMSGSIZE;
> >
> > --
> > 2.25.1
>