linux-kernel - Re: [PATCH V6 8/8] virtio: Intel IFC VF driver for VDPA

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Message-ID: <055f10a8-0b7d-ec2b-4fd4-d47ba96ddd5f@redhat.com>
Date:   Fri, 20 Mar 2020 17:17:31 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     mst@...hat.com, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        maxime.coquelin@...hat.com, cunming.liang@...el.com,
        zhihong.wang@...el.com, rob.miller@...adcom.com,
        xiao.w.wang@...el.com, lingshan.zhu@...el.com, eperezma@...hat.com,
        lulu@...hat.com, parav@...lanox.com, kevin.tian@...el.com,
        stefanha@...hat.com, rdunlap@...radead.org, hch@...radead.org,
        aadam@...hat.com, jiri@...lanox.com, shahafs@...lanox.com,
        hanand@...inx.com, mhabets@...arflare.com, gdawar@...inx.com,
        saugatm@...inx.com, vmireyno@...vell.com,
        Bie Tiwei <tiwei.bie@...el.com>
Subject: Re: [PATCH V6 8/8] virtio: Intel IFC VF driver for VDPA


On 2020/3/19 下午9:02, Jason Gunthorpe wrote:
> On Thu, Mar 19, 2020 at 04:14:37PM +0800, Jason Wang wrote:
>> On 2020/3/18 下午8:22, Jason Gunthorpe wrote:
>>> On Wed, Mar 18, 2020 at 04:03:27PM +0800, Jason Wang wrote:
>>>> From: Zhu Lingshan <lingshan.zhu@...el.com>
>>>> +
>>>> +static int ifcvf_vdpa_attach(struct ifcvf_adapter *adapter)
>>>> +{
>>>> +	int ret;
>>>> +
>>>> +	adapter->vdpa_dev  = vdpa_alloc_device(adapter->dev, adapter->dev,
>>>> +					       &ifc_vdpa_ops);
>>>> +	if (IS_ERR(adapter->vdpa_dev)) {
>>>> +		IFCVF_ERR(adapter->dev, "Failed to init ifcvf on vdpa bus");
>>>> +		put_device(&adapter->vdpa_dev->dev);
>>>> +		return -ENODEV;
>>>> +	}
>>> The point of having an alloc call is so that the drivers
>>> ifcvf_adaptor memory could be placed in the same struct - eg use
>>> container_of to flip between them, and have a kref for both memories.
>>>
>>> It seem really weird to have an alloc followed immediately by
>>> register.
>>
>> I admit the ifcvf_adapter is not correctly ref-counted. What you suggest
>> should work. But it looks to me the following is more cleaner since the
>> members of ifcvf_adapter are all related to PCI device not vDPA itself.
> I've done it both ways (eg tpm is as you describe, ib is using alloc).
>
> I tend to prefer the alloc method today, allowing the driver memory to
> have a proper refcount makes the driver structure usable with RCU and
> allows simple solutions to some tricky cases. It is a bit hard to
> switch to this later..
>
>> - keep the current layout of ifcvf_adapter
>> - merge vdpa_alloc_device() and vdpa_register_device()
>> - use devres to bind ifcvf_adapter refcnt/lifcycle to the under PCI device
> This is almost what tpm does. Keep in mind the lifecycle with devm is
> just slightly past the driver remove call, so remove still
> must revoke all external references to the memory.
>
> The merging alloc and register rarely works out, the register must be
> the very last thing done, and usually you need the subsystem pointer
> to do pre-registration setup in anything but the most trivial of
> subsystems and drivers.
>
>> If we go for the container_of method, we probably need
>>
>> - accept a size of parent parent structure in vdpa_alloc_device() and
>> mandate vdpa_device to be the first member of ifcvf_adapter
>> - we need provide a way to free resources of parent structure when we
>> destroy vDPA device
> Yep. netdev and rdma work this way with a free memory callback in the
> existing ops structures.
>
> Jason


Ok, I get your points now. Will go for way of container_of in next version.

Thanks