回复: FW: [PATCH] vhost-user: add VIRTIO_F_IN_ORDER and VIRTIO_F_NOTIFICATION_DATA feature

[Rick Zhong]

Hi Eugenio,

Thanks for your comments. Very helpful. Wentao and I will discuss and get back 
to you later.

Also welcome for any comments from other guys.

Best Regards,
Rick Zhong

-----邮件原件-----
发件人: Eugenio Perez Martin <eperezma@redhat.com> 
发送时间: 2024年1月19日 18:26
收件人: Wentao Jia <wentao.jia@nephogine.com>
抄送: qemu-devel@nongnu.org; mst@redhat.com; Rick Zhong 
<zhaoyong.zhong@nephogine.com>; Jason Wang <jasowang@redhat.com>; Peter Xu 
<peterx@redhat.com>; Guo Zhi <qtxuning1999@sjtu.edu.cn>
主题: Re: FW: [PATCH] vhost-user: add VIRTIO_F_IN_ORDER and 
VIRTIO_F_NOTIFICATION_DATA feature

On Fri, Jan 19, 2024 at 7:42 AM Wentao Jia <wentao.jia@nephogine.com> wrote:
>
>
> VIRTIO_F_IN_ORDER and VIRTIO_F_NOTIFICATION_DATA feature are important 
> feature for dpdk vdpa packets transmitting performance, add the 2 
> features at vhost-user front-end to negotiation with backend.
>
> Signed-off-by: Kyle Xu <zhenbing.xu@corigine.com>
> Signed-off-by: Wentao Jia <wentao.jia@corigine.com>
> Reviewed-by:   Xinying Yu <xinying.yu@corigine.com>
> Reviewed-by:   Shujing Dong <shujing.dong@corigine.com>
> Reviewed-by:   Rick Zhong <zhaoyong.zhong@corigine.com>
> ---
>  hw/core/machine.c   | 2 ++
>  hw/net/vhost_net.c  | 2 ++
>  hw/net/virtio-net.c | 4 ++++
>  3 files changed, 8 insertions(+)
>
> diff --git a/hw/core/machine.c b/hw/core/machine.c index 
> fb5afdcae4..e620f5e7d0 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -40,6 +40,7 @@ GlobalProperty hw_compat_8_1[] = {
>      { "ramfb", "x-migrate", "off" },
>      { "vfio-pci-nohotplug", "x-ramfb-migrate", "off" },
>      { "igb", "x-pcie-flr-init", "off" },
> +    { TYPE_VIRTIO_NET, "notification_data", "off"},
>  };

Assuming the default "true" in
hw/net/virtio-net.c:virtio_net_properties is valid, this needs to be appended 
to the array of the QEMU version that introduced the property in the 
virtio_net_properties array, not the one that imported the macro from the 
kernel. This allows QEMU to know that old versions have these features disabled 
although the default set in hw/net/virtio-net.c:virtio_net_properties is true 
when migrating from / to these versions.

You can check that this is added properly by migrating from / to a previous 
version of QEMU, with the combinations of true and false.

You have an example in [1] with blk devices multiqueue. CCing Peter Xu as he 
knows more than me about this.

This is very easy to miss when adding new features. Somebody who knows perl 
should add a test in checkpath.pl similar to the warning "added, moved or 
deleted file(s), does MAINTAINERS need updating?" when virtio properties are 
modified :).

>  const size_t hw_compat_8_1_len = G_N_ELEMENTS(hw_compat_8_1);
>
> @@ -65,6 +66,7 @@ GlobalProperty hw_compat_7_1[] = {
>      { "virtio-rng-pci", "vectors", "0" },
>      { "virtio-rng-pci-transitional", "vectors", "0" },
>      { "virtio-rng-pci-non-transitional", "vectors", "0" },
> +    { TYPE_VIRTIO_NET, "in_order", "off"},
>  };
>  const size_t hw_compat_7_1_len = G_N_ELEMENTS(hw_compat_7_1);
>
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c index 
> e8e1661646..211ca859a6 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -76,6 +76,8 @@ static const int user_feature_bits[] = {
>      VIRTIO_F_IOMMU_PLATFORM,
>      VIRTIO_F_RING_PACKED,
>      VIRTIO_F_RING_RESET,
> +    VIRTIO_F_IN_ORDER,
> +    VIRTIO_F_NOTIFICATION_DATA,
>      VIRTIO_NET_F_RSS,
>      VIRTIO_NET_F_HASH_REPORT,
>      VIRTIO_NET_F_GUEST_USO4,
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c index 
> 7a2846fa1c..dc0a028934 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -3949,6 +3949,10 @@ static Property virtio_net_properties[] = {
>                        VIRTIO_NET_F_GUEST_USO6, true),
>      DEFINE_PROP_BIT64("host_uso", VirtIONet, host_features,
>                        VIRTIO_NET_F_HOST_USO, true),
> +    DEFINE_PROP_BIT64("in_order", VirtIONet, host_features,
> +                      VIRTIO_F_IN_ORDER, true),
> +    DEFINE_PROP_BIT64("notification_data", VirtIONet, host_features,
> +                      VIRTIO_F_NOTIFICATION_DATA, true),

This default=true is wrong, and makes emulated devices show these features as 
available when they're not. You can test it by running qemu with the parameters:

-netdev tap,id=hostnet0,vhost=off -device virtio-net-pci,netdev=hostnet0,...

The emulated device must support both features before making them tunnables.

On the other hand, all kinds of virtio devices can use in_order and 
notification_data, so they should be in 
include/hw/virtio/virtio.h:DEFINE_VIRTIO_COMMON_FEATURES. But not all of them 
benefit from in_order. One example of this is virtio-blk. It is usual that 
requests are completed out of order by the backend device, so my impression is 
that in_order will hurt its performance.
I've never profiled it though, so I may be wrong :).

Long story short: Maybe in_order should be false by default, and enabled just 
in virtio-net?

You can see previous attempts of implementing this feature in qemu in [2]. 
CCing Guo too, as I don't know if he plans to continue this work soon.

Please let me know if you need any help with these!

Thanks!

[1] 
https://www.qemu.org/docs/master/devel/migration/compatibility.html#how-backwards-compatibility-works
[2] https://lists.gnu.org/archive/html/qemu-devel/2022-08/msg02772.html

>      DEFINE_PROP_END_OF_LIST(),
>  };
>
> --
>