Re: [External] Re: [RFC v1] virtio/vsock: add two more queues for datagram types

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, Jun 30, 2021 at 03:44:17PM -0700, Jiang Wang . wrote:
>On Thu, Jun 24, 2021 at 7:31 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
>>
>> On Wed, Jun 23, 2021 at 11:50:33PM -0700, Jiang Wang . wrote:
>> >Hi Stefano,
>> >
>> >I checked virtio_net_set_multiqueue(), which will help with following
>> >changes in my patch:
>> >
>> >#ifdef CONFIG_VHOST_VSOCK_DGRAM
>> >vvc->dgram_recv_vq = virtio_add_queue(vdev, VHOST_VSOCK_QUEUE_SIZE,
>> >vhost_vsock_common_handle_output);
>> >vvc->dgram_trans_vq = virtio_add_queue(vdev, VHOST_VSOCK_QUEUE_SIZE,
>> >vhost_vsock_common_handle_output);
>> >#endif
>> >
>> >But I think there is still an issue with the following lines, right?
>>
>> Yep, I think so.
>>
>> >
>> >#ifdef CONFIG_VHOST_VSOCK_DGRAM
>> >struct vhost_virtqueue vhost_vqs[4];
>> >#else
>> >struct vhost_virtqueue vhost_vqs[2];
>> >#endif
>> >
>> >I think the problem with feature bits is that they are set and get after
>> >vhost_vsock_common_realize() and after vhost_dev_init() in drivers/vhost/vsock.c
>> >But those virtqueues need to be set up correctly beforehand.
>>
>> I think we can follow net and scsi vhost devices, so we can set a
>> VHOST_VSOCK_VQ_MAX(5), allocates all the queues in any case and then use
>> only the queues acked by the guest.
>>
>Thanks for the advice. I checked both net and scsi and scsi is more helpful.

Yeah :-)

>
>> >
>> >I tried to test with the host kernel allocating 4 vqs, but qemu only
>> >allocated 2 vqs, and
>> >guest kernel will not be able to send even the vsock stream packets. I
>> >think the host
>> >kernel and the qemu have to agree on the number of vhost_vqs. Do you agree?
>> >Did I miss something?
>>
>> Mmm, I need to check, but for example vhost-net calls vhost_dev_init()
>> with VHOST_NET_VQ_MAX, but then the guest can decide to use only one
>> couple of TX and RX queues.
>>
>> I'm not sure about qemu point of view, but I expected that QEMU can set
>> less queues then queues allocated by the kernel. `vhost_dev.nvqs` should
>> be set with the amount of queue that QEMU can handle.
>>
>I checked that vhost_dev.nvqs is still the maximum number of queues (4 queues).
>But I found a way to workaround it. More details in the following text.
>
>> >
>> >Another idea to make the setting in runtime instead of compiling time
>> >is to use
>> >qemu cmd-line options, then qemu can allocate 2 or 4 queues depending
>> >on
>> >the cmd line. This will solve the issue when the host kernel is an old
>> >one( no dgram
>> >support) and the qemu is a new one.
>>
>> I don't think this is a good idea, at most we can add an ioctl that qemu
>> can use to query the kernel about allocated queues, but I still need to
>> understand better if we really we need this.
>>
>
>Hmm. Both net and scsi use the qemu cmd line option to configure
>number of queues. Qemu cmdline is a runtime setting and flexible.
>I think qemu cmdline is better than ioctl. I also make the qemu cmd
>line option default to only allocate two queues to be compatible with
>old versions.

Can we avoid both and allocate the maximum number of queue that QEMU can 
handle?

I'm not sure that adding a parameter to QEMU is a good idea. If possible 
it should be automatic.

>
>> >
>> >But there is still an issue when the host kernel is a new one, while
>> >the qemu
>> >is an old one.  I am not sure how to make the virtqueues numbers to
>> >change in run-time
>> >for the host kernel. In another email thread, you mentioned removing 
>> >kconfig
>> >in the linux kernel, I believe that is related to this qemu patch,
>> >right?
>>
>> It was related to both, I don't think we should build QEMU and Linux
>> with or without dgram support.
>>
>> > If so,
>> >any ideas that I can make the host kernel change the number of vqs 
>> >in
>> >the run-time
>> >or when starting up vsock? The only way I can think of is to use a
>> >kernel module parameter
>> >for the vsock_vhost module. Any other ideas? Thanks.
>>
>> I need to check better, but we should be able to do all at run time
>> looking at the features field. As I said, both QEMU and kernel can
>> allocate the maximum number of queues that they can handle, then 
>> enable
>> only the queues allocated by the guest (e.g. during
>> vhost_vsock_common_start()).
>>
>
>Yes. I checked the code and found there is an implementation bug ( or
>limitation) in drivers/vhost/vsock.c. In vhost_vsock_start(), if a 
>queue
>failed to init, the code will clean up all previous successfully
>allocated queues. That is why V1 code does not work when
>host kernel is new,  but qemu and guest kernel is old. I made a change
>there and it works now. I will clean up the patch a little bit and
>send V2 soon.

Great! I'll review the new version!

Thanks,
Stefano