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

[Jiang Wang .]

On Wed, Jun 30, 2021 at 11:55 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>
> 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 did not find any way to do that. When the host kernel is new, QEMU
can handle 4 queues. When the host kernel is an old one, QEMU can
only handle 2 queues.

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

OK. I will keep that in mind and dig more to see if there is a way to
do that.

Thanks.

> >
> >> >
> >> >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
>