netdev - Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F

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Date:   Wed, 27 Jul 2022 14:56:20 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Parav Pandit <parav@...dia.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xieyongji@...edance.com" <xieyongji@...edance.com>,
        "gautam.dawar@....com" <gautam.dawar@....com>
Subject: Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace
 when VIRTIO_NET_F_MQ == 0

On Wed, Jul 27, 2022 at 2:26 PM Zhu, Lingshan <lingshan.zhu@...el.com> wrote:
>
>
>
> On 7/27/2022 2:01 PM, Michael S. Tsirkin wrote:
> > On Wed, Jul 27, 2022 at 03:47:35AM +0000, Parav Pandit wrote:
> >>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> >>> Sent: Tuesday, July 26, 2022 10:53 PM
> >>>
> >>> On 7/27/2022 10:17 AM, Parav Pandit wrote:
> >>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> >>>>> Sent: Tuesday, July 26, 2022 10:15 PM
> >>>>>
> >>>>> On 7/26/2022 11:56 PM, Parav Pandit wrote:
> >>>>>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> >>>>>>> Sent: Tuesday, July 12, 2022 11:46 PM
> >>>>>>>> When the user space which invokes netlink commands, detects that
> >>>>> _MQ
> >>>>>>> is not supported, hence it takes max_queue_pair = 1 by itself.
> >>>>>>> I think the kernel module have all necessary information and it is
> >>>>>>> the only one which have precise information of a device, so it
> >>>>>>> should answer precisely than let the user space guess. The kernel
> >>>>>>> module should be reliable than stay silent, leave the question to
> >>>>>>> the user space
> >>>>> tool.
> >>>>>> Kernel is reliable. It doesn’t expose a config space field if the
> >>>>>> field doesn’t
> >>>>> exist regardless of field should have default or no default.
> >>>>> so when you know it is one queue pair, you should answer one, not try
> >>>>> to guess.
> >>>>>> User space should not guess either. User space gets to see if _MQ
> >>>>> present/not present. If _MQ present than get reliable data from kernel.
> >>>>>> If _MQ not present, it means this device has one VQ pair.
> >>>>> it is still a guess, right? And all user space tools implemented this
> >>>>> feature need to guess
> >>>> No. it is not a guess.
> >>>> It is explicitly checking the _MQ feature and deriving the value.
> >>>> The code you proposed will be present in the user space.
> >>>> It will be uniform for _MQ and 10 other features that are present now and
> >>> in the future.
> >>> MQ and other features like RSS are different. If there is no _RSS_XX, there
> >>> are no attributes like max_rss_key_size, and there is not a default value.
> >>> But for MQ, we know it has to be 1 wihtout _MQ.
> >> "we" = user space.
> >> To keep the consistency among all the config space fields.
> > Actually I looked and the code some more and I'm puzzled:
> I can submit a fix in my next version patch for these issue.
> >
> >
> >       struct virtio_net_config config = {};
> >       u64 features;
> >       u16 val_u16;
> >
> >       vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
> >
> >       if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, sizeof(config.mac),
> >                   config.mac))
> >               return -EMSGSIZE;
> >
> >
> > Mac returned even without VIRTIO_NET_F_MAC
> if no VIRTIO_NET_F_MAC, we should not nla_put
> VDPA_ATTR_DEV_NET_CFG_MAC_ADDR, the spec says the driver should generate
> a random mac.

It's probably too late to do this. Most of the parents have this
feature support, so probably not a real issue.

> >
> >
> >       val_u16 = le16_to_cpu(config.status);
> >       if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
> >               return -EMSGSIZE;
> >
> >
> > status returned even without VIRTIO_NET_F_STATUS
> if no VIRTIO_NET_F_STATUS, we should not nla_put
> VDPA_ATTR_DEV_NET_STATUS, the spec says the driver should assume the
> link is active.

Somehow similar to F_MAC. But we can report if F_MAC is not negotiated.


> >
> >       val_u16 = le16_to_cpu(config.mtu);
> >       if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
> >               return -EMSGSIZE;
> >
> >
> > MTU returned even without VIRTIO_NET_F_MTU
> same as above, the spec says config.mtu depends on VIRTIO_NET_F_MTU, so
> without this feature bit, we should not return MTU to the userspace.

Not a big issue, we just need to make sure the parent can report a
correct MTU here.

Thanks

>
> Does these fix look good to you?
>
> And I think we may need your adjudication for the two issues:
> (1) Shall we answer max_vq_paris = 1 when _MQ not exist, I know you have
> agreed on this in a previous thread, its nice to clarify
> (2) I think we should not re-use the netlink attr to report feature bits
> of both the management device and the vDPA device,
> this can lead to a new race condition, there are no locks(especially
> distributed locks for kernel_space and user_space) in the nla_put
> functions. Re-using the attr is some kind of breaking the netlink
> lockless design.
>
> Thanks,
> Zhu Lingshan
> >
> >
> > What's going on here?
> >
> >
>