Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace

From: Jason Wang <jasowang@redhat.com>
To: Yongji Xie <xieyongji@bytedance.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	"Stefan Hajnoczi" <stefanha@redhat.com>,
	"Stefano Garzarella" <sgarzare@redhat.com>,
	"Parav Pandit" <parav@nvidia.com>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Christian Brauner" <christian.brauner@canonical.com>,
	"Randy Dunlap" <rdunlap@infradead.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Jens Axboe" <axboe@kernel.dk>,
	bcrl@kvack.org, "Jonathan Corbet" <corbet@lwn.net>,
	"Mika Penttilä" <mika.penttila@nextfour.com>,
	"Dan Carpenter" <dan.carpenter@oracle.com>,
	joro@8bytes.org, "Greg KH" <gregkh@linuxfoundation.org>,
	songmuchun@bytedance.com,
	virtualization <virtualization@lists.linux-foundation.org>,
	netdev@vger.kernel.org, kvm <kvm@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, iommu@lists.linux-foundation.org,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace
Date: Tue, 29 Jun 2021 12:03:25 +0800	[thread overview]
Message-ID: <d30e391f-a900-5182-f732-e7c0089b7cbd@redhat.com> (raw)
In-Reply-To: <CACycT3ucVz3D4Tcr1C6uzWyApZy7Xk4o17VH2gvLO3w1Ra+skg@mail.gmail.com>

在 2021/6/29 上午11:56, Yongji Xie 写道:
> On Tue, Jun 29, 2021 at 11:29 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 在 2021/6/29 上午10:26, Yongji Xie 写道:
>>> On Mon, Jun 28, 2021 at 12:40 PM Jason Wang <jasowang@redhat.com> wrote:
>>>> 在 2021/6/25 下午12:19, Yongji Xie 写道:
>>>>>> 2b) for set_status(): simply relay the message to userspace, reply is no
>>>>>> needed. Userspace will use a command to update the status when the
>>>>>> datapath is stop. The the status could be fetched via get_stats().
>>>>>>
>>>>>> 2b looks more spec complaint.
>>>>>>
>>>>> Looks good to me. And I think we can use the reply of the message to
>>>>> update the status instead of introducing a new command.
>>>>>
>>>> Just notice this part in virtio_finalize_features():
>>>>
>>>>            virtio_add_status(dev, VIRTIO_CONFIG_S_FEATURES_OK);
>>>>            status = dev->config->get_status(dev);
>>>>            if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>>>>
>>>> So we no reply doesn't work for FEATURES_OK.
>>>>
>>>> So my understanding is:
>>>>
>>>> 1) We must not use noreply for set_status()
>>>> 2) We can use noreply for get_status(), but it requires a new ioctl to
>>>> update the status.
>>>>
>>>> So it looks to me we need synchronize for both get_status() and
>>>> set_status().
>>>>
>>> We should not send messages to userspace in the FEATURES_OK case. So
>>> the synchronization is not necessary.
>>
>> As discussed previously, there could be a device that mandates some
>> features (VIRTIO_F_RING_PACKED). So it can choose to not accept
>> FEATURES_OK is packed virtqueue is not negotiated.
>>
>> In this case we need to relay the message to userspace.
>>
> OK, I see. If so, I prefer to only use noreply for set_status(). We do
> not set the status bit if the message is failed. In this way, we don't
> need to change lots of virtio core codes to handle the failure of
> set_status()/get_status().

It should work.

Thanks

>
> Thanks,
> Yongji
>