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Date:   Tue, 9 Aug 2022 19:27:11 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "mst@...hat.com" <mst@...hat.com>
CC:     "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 V4 3/6] vDPA: allow userspace to query features of a vDPA
 device

> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> Sent: Wednesday, July 27, 2022 2:02 AM
> 
> 
> On 7/26/2022 7:06 PM, Parav Pandit wrote:
> >> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> >> Sent: Tuesday, July 26, 2022 7:03 AM
> >>
> >> On 7/24/2022 11:21 PM, Parav Pandit wrote:
> >>>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> >>>> Sent: Saturday, July 23, 2022 7:24 AM
> >>>>
> >>>>
> >>>> On 7/22/2022 9:12 PM, Parav Pandit wrote:
> >>>>>> From: Zhu Lingshan <lingshan.zhu@...el.com>
> >>>>>> Sent: Friday, July 22, 2022 7:53 AM
> >>>>>>
> >>>>>> This commit adds a new vDPA netlink attribution
> >>>>>> VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES. Userspace can
> >> query
> >>>> features
> >>>>>> of vDPA devices through this new attr.
> >>>>>>
> >>>>>> Signed-off-by: Zhu Lingshan <lingshan.zhu@...el.com>
> >>>>>> ---
> >>>>>>     drivers/vdpa/vdpa.c       | 13 +++++++++----
> >>>>>>     include/uapi/linux/vdpa.h |  1 +
> >>>>>>     2 files changed, 10 insertions(+), 4 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c index
> >>>>>> ebf2f363fbe7..9b0e39b2f022 100644
> >>>>>> --- a/drivers/vdpa/vdpa.c
> >>>>>> +++ b/drivers/vdpa/vdpa.c
> >>>>>> @@ -815,7 +815,7 @@ static int
> vdpa_dev_net_mq_config_fill(struct
> >>>>>> vdpa_device *vdev,  static int vdpa_dev_net_config_fill(struct
> >>>>>> vdpa_device *vdev, struct sk_buff *msg)  {
> >>>>>>     	struct virtio_net_config config = {};
> >>>>>> -	u64 features;
> >>>>>> +	u64 features_device, features_driver;
> >>>>>>     	u16 val_u16;
> >>>>>>
> >>>>>>     	vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
> >>>>>> @@
> >>>>>> -
> >>>>>> 832,12 +832,17 @@ static int vdpa_dev_net_config_fill(struct
> >>>>>> vdpa_device *vdev, struct sk_buff *ms
> >>>>>>     	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU,
> val_u16))
> >>>>>>     		return -EMSGSIZE;
> >>>>>>
> >>>>>> -	features = vdev->config->get_driver_features(vdev);
> >>>>>> -	if (nla_put_u64_64bit(msg,
> >>>>>> VDPA_ATTR_DEV_NEGOTIATED_FEATURES, features,
> >>>>>> +	features_driver = vdev->config->get_driver_features(vdev);
> >>>>>> +	if (nla_put_u64_64bit(msg,
> >>>>>> VDPA_ATTR_DEV_NEGOTIATED_FEATURES, features_driver,
> >>>>>> +			      VDPA_ATTR_PAD))
> >>>>>> +		return -EMSGSIZE;
> >>>>>> +
> >>>>>> +	features_device = vdev->config->get_device_features(vdev);
> >>>>>> +	if (nla_put_u64_64bit(msg,
> >>>>>> VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES,
> >>>>>> +features_device,
> >>>>>>     			      VDPA_ATTR_PAD))
> >>>>>>     		return -EMSGSIZE;
> >>>>>>
> >>>>>> -	return vdpa_dev_net_mq_config_fill(vdev, msg, features,
> &config);
> >>>>>> +	return vdpa_dev_net_mq_config_fill(vdev, msg,
> features_driver,
> >>>>>> +&config);
> >>>>>>     }
> >>>>>>
> >>>>>>     static int
> >>>>>> diff --git a/include/uapi/linux/vdpa.h
> >>>>>> b/include/uapi/linux/vdpa.h index
> >>>>>> 25c55cab3d7c..39f1c3d7c112 100644
> >>>>>> --- a/include/uapi/linux/vdpa.h
> >>>>>> +++ b/include/uapi/linux/vdpa.h
> >>>>>> @@ -47,6 +47,7 @@ enum vdpa_attr {
> >>>>>>     	VDPA_ATTR_DEV_NEGOTIATED_FEATURES,	/* u64 */
> >>>>>>     	VDPA_ATTR_DEV_MGMTDEV_MAX_VQS,		/*
> u32 */
> >>>>>>     	VDPA_ATTR_DEV_SUPPORTED_FEATURES,	/* u64 */
> >>>>>> +	VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES,	/*
> u64 */
> >>>>>>
> >>>>> I have answered in previous emails.
> >>>>> I disagree with the change.
> >>>>> Please reuse VDPA_ATTR_DEV_SUPPORTED_FEATURES.
> >>>> I believe we have already discussed this before in the V3 thread.
> >>>> I have told you that reusing this attr will lead to a new race condition.
> >>>>
> >>> Returning attribute cannot lead to any race condition.
> >> Please refer to our discussion in the V3 series, I have explained if
> >> re-use this attr, it will be a multiple consumers and multiple
> >> produces model, it is a typical racing condition.
> > I read the emails with subject = " Re: [PATCH V3 3/6] vDPA: allow userspace
> to query features of a vDPA device"
> > I couldn’t find multiple consumers multiple producers working on same nla
> message.
> If this attr is reused, then there can be multiple iproute2 instances or other
> applications querying feature bits of the management device and the vDPA
> device simultaneously, and both kernel side management feature bits filler

> function and vDPA device feature bits filler function can write the NLA
> message at the same time. That's the multiple consumers and producers,
> and no locks
No. Each filling up happens in each process context. There is no race here.

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