From: Jason Wang <jasowang@redhat.com>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Yongji Xie" <xieyongji@bytedance.com>,
"Michael S. Tsirkin" <mst@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 10/10] Documentation: Add documentation for VDUSE
Date: Tue, 6 Jul 2021 10:34:33 +0800 [thread overview]
Message-ID: <5b5107fa-3b32-8a3b-720d-eee6b2a84ace@redhat.com> (raw)
In-Reply-To: <YOL/9mxkJaokKDHc@stefanha-x1.localdomain>
在 2021/7/5 下午8:49, Stefan Hajnoczi 写道:
> On Mon, Jul 05, 2021 at 11:36:15AM +0800, Jason Wang wrote:
>> 在 2021/7/4 下午5:49, Yongji Xie 写道:
>>>>> OK, I get you now. Since the VIRTIO specification says "Device
>>>>> configuration space is generally used for rarely-changing or
>>>>> initialization-time parameters". I assume the VDUSE_DEV_SET_CONFIG
>>>>> ioctl should not be called frequently.
>>>> The spec uses MUST and other terms to define the precise requirements.
>>>> Here the language (especially the word "generally") is weaker and means
>>>> there may be exceptions.
>>>>
>>>> Another type of access that doesn't work with the VDUSE_DEV_SET_CONFIG
>>>> approach is reads that have side-effects. For example, imagine a field
>>>> containing an error code if the device encounters a problem unrelated to
>>>> a specific virtqueue request. Reading from this field resets the error
>>>> code to 0, saving the driver an extra configuration space write access
>>>> and possibly race conditions. It isn't possible to implement those
>>>> semantics suing VDUSE_DEV_SET_CONFIG. It's another corner case, but it
>>>> makes me think that the interface does not allow full VIRTIO semantics.
>>
>> Note that though you're correct, my understanding is that config space is
>> not suitable for this kind of error propagating. And it would be very hard
>> to implement such kind of semantic in some transports. Virtqueue should be
>> much better. As Yong Ji quoted, the config space is used for
>> "rarely-changing or intialization-time parameters".
>>
>>
>>> Agreed. I will use VDUSE_DEV_GET_CONFIG in the next version. And to
>>> handle the message failure, I'm going to add a return value to
>>> virtio_config_ops.get() and virtio_cread_* API so that the error can
>>> be propagated to the virtio device driver. Then the virtio-blk device
>>> driver can be modified to handle that.
>>>
>>> Jason and Stefan, what do you think of this way?
> Why does VDUSE_DEV_GET_CONFIG need to support an error return value?
>
> The VIRTIO spec provides no way for the device to report errors from
> config space accesses.
>
> The QEMU virtio-pci implementation returns -1 from invalid
> virtio_config_read*() and silently discards virtio_config_write*()
> accesses.
>
> VDUSE can take the same approach with
> VDUSE_DEV_GET_CONFIG/VDUSE_DEV_SET_CONFIG.
>
>> I'd like to stick to the current assumption thich get_config won't fail.
>> That is to say,
>>
>> 1) maintain a config in the kernel, make sure the config space read can
>> always succeed
>> 2) introduce an ioctl for the vduse usersapce to update the config space.
>> 3) we can synchronize with the vduse userspace during set_config
>>
>> Does this work?
> I noticed that caching is also allowed by the vhost-user protocol
> messages (QEMU's docs/interop/vhost-user.rst), but the device doesn't
> know whether or not caching is in effect. The interface you outlined
> above requires caching.
>
> Is there a reason why the host kernel vDPA code needs to cache the
> configuration space?
Because:
1) Kernel can not wait forever in get_config(), this is the major
difference with vhost-user.
2) Stick to the current assumption that virtio_cread() should always
succeed.
Thanks
>
> Here are the vhost-user protocol messages:
>
> Virtio device config space
> ^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> +--------+------+-------+---------+
> | offset | size | flags | payload |
> +--------+------+-------+---------+
>
> :offset: a 32-bit offset of virtio device's configuration space
>
> :size: a 32-bit configuration space access size in bytes
>
> :flags: a 32-bit value:
> - 0: Vhost master messages used for writeable fields
> - 1: Vhost master messages used for live migration
>
> :payload: Size bytes array holding the contents of the virtio
> device's configuration space
>
> ...
>
> ``VHOST_USER_GET_CONFIG``
> :id: 24
> :equivalent ioctl: N/A
> :master payload: virtio device config space
> :slave payload: virtio device config space
>
> When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
> submitted by the vhost-user master to fetch the contents of the
> virtio device configuration space, vhost-user slave's payload size
> MUST match master's request, vhost-user slave uses zero length of
> payload to indicate an error to vhost-user master. The vhost-user
> master may cache the contents to avoid repeated
> ``VHOST_USER_GET_CONFIG`` calls.
>
> ``VHOST_USER_SET_CONFIG``
> :id: 25
> :equivalent ioctl: N/A
> :master payload: virtio device config space
> :slave payload: N/A
>
> When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
> submitted by the vhost-user master when the Guest changes the virtio
> device configuration space and also can be used for live migration
> on the destination host. The vhost-user slave must check the flags
> field, and slaves MUST NOT accept SET_CONFIG for read-only
> configuration space fields unless the live migration bit is set.
>
> Stefan
next prev parent reply other threads:[~2021-07-06 2:34 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-15 14:13 [PATCH v8 00/10] Introduce VDUSE - vDPA Device in Userspace Xie Yongji
2021-06-15 14:13 ` [PATCH v8 01/10] iova: Export alloc_iova_fast() and free_iova_fast(); Xie Yongji
2021-06-15 14:13 ` [PATCH v8 02/10] file: Export receive_fd() to modules Xie Yongji
2021-06-15 14:13 ` [PATCH v8 03/10] eventfd: Increase the recursion depth of eventfd_signal() Xie Yongji
2021-06-17 8:33 ` He Zhe
2021-06-18 3:29 ` Yongji Xie
2021-06-18 8:41 ` He Zhe
2021-06-18 8:44 ` [PATCH] eventfd: Enlarge recursion limit to allow vhost to work He Zhe
2021-07-03 8:31 ` Michael S. Tsirkin
2021-08-25 7:57 ` Yongji Xie
2021-06-15 14:13 ` [PATCH v8 04/10] vhost-iotlb: Add an opaque pointer for vhost IOTLB Xie Yongji
2021-06-15 14:13 ` [PATCH v8 05/10] vdpa: Add an opaque pointer for vdpa_config_ops.dma_map() Xie Yongji
2021-06-15 14:13 ` [PATCH v8 06/10] vdpa: factor out vhost_vdpa_pa_map() and vhost_vdpa_pa_unmap() Xie Yongji
2021-06-15 14:13 ` [PATCH v8 07/10] vdpa: Support transferring virtual addressing during DMA mapping Xie Yongji
2021-06-15 14:13 ` [PATCH v8 08/10] vduse: Implement an MMU-based IOMMU driver Xie Yongji
2021-06-15 14:13 ` [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace Xie Yongji
2021-06-21 9:13 ` Jason Wang
2021-06-21 10:41 ` Yongji Xie
2021-06-22 5:06 ` Jason Wang
2021-06-22 7:22 ` Yongji Xie
2021-06-22 7:49 ` Jason Wang
2021-06-22 8:14 ` Yongji Xie
2021-06-23 3:30 ` Jason Wang
2021-06-23 5:50 ` Yongji Xie
2021-06-24 3:34 ` Jason Wang
2021-06-24 4:46 ` Yongji Xie
2021-06-24 8:13 ` Jason Wang
2021-06-24 9:16 ` Yongji Xie
2021-06-25 3:08 ` Jason Wang
2021-06-25 4:19 ` Yongji Xie
2021-06-28 4:40 ` Jason Wang
2021-06-29 2:26 ` Yongji Xie
2021-06-29 3:29 ` Jason Wang
2021-06-29 3:56 ` Yongji Xie
2021-06-29 4:03 ` Jason Wang
2021-06-24 14:46 ` Stefan Hajnoczi
2021-06-29 2:59 ` Yongji Xie
2021-06-30 9:51 ` Stefan Hajnoczi
2021-07-01 6:50 ` Yongji Xie
2021-07-01 7:55 ` Jason Wang
2021-07-01 10:26 ` Yongji Xie
2021-07-02 3:25 ` Jason Wang
2021-07-07 8:52 ` Stefan Hajnoczi
2021-07-07 9:19 ` Yongji Xie
2021-06-15 14:13 ` [PATCH v8 10/10] Documentation: Add documentation for VDUSE Xie Yongji
2021-06-24 13:01 ` Stefan Hajnoczi
2021-06-29 5:43 ` Yongji Xie
2021-06-30 10:06 ` Stefan Hajnoczi
2021-07-01 10:00 ` Yongji Xie
2021-07-01 13:15 ` Stefan Hajnoczi
2021-07-04 9:49 ` Yongji Xie
2021-07-05 3:36 ` Jason Wang
2021-07-05 12:49 ` Stefan Hajnoczi
2021-07-06 2:34 ` Jason Wang [this message]
2021-07-06 10:14 ` Stefan Hajnoczi
[not found] ` <CACGkMEs2HHbUfarum8uQ6wuXoDwLQUSXTsa-huJFiqr__4cwRg@mail.gmail.com>
[not found] ` <YOSOsrQWySr0andk@stefanha-x1.localdomain>
[not found] ` <100e6788-7fdf-1505-d69c-bc28a8bc7a78@redhat.com>
[not found] ` <YOVr801d01YOPzLL@stefanha-x1.localdomain>
2021-07-07 9:24 ` Jason Wang
2021-07-07 15:54 ` Stefan Hajnoczi
2021-07-08 4:17 ` Jason Wang
2021-07-08 9:06 ` Stefan Hajnoczi
2021-07-08 12:35 ` Yongji Xie
2021-07-06 3:04 ` Yongji Xie
2021-07-06 10:22 ` Stefan Hajnoczi
2021-07-07 9:09 ` Yongji Xie
2021-07-08 9:07 ` Stefan Hajnoczi
2021-06-24 15:12 ` [PATCH v8 00/10] Introduce VDUSE - vDPA Device in Userspace Stefan Hajnoczi
2021-06-29 3:15 ` Yongji Xie
2021-06-28 10:33 ` Liu Xiaodong
2021-06-28 4:35 ` Jason Wang
2021-06-28 5:54 ` Liu, Xiaodong
2021-06-29 4:10 ` Jason Wang
2021-06-29 7:56 ` Liu, Xiaodong
2021-06-29 8:14 ` Yongji Xie
2021-06-28 10:32 ` Yongji Xie
2021-06-29 4:12 ` Jason Wang
2021-06-29 6:40 ` Yongji Xie
2021-06-29 7:33 ` Jason Wang
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