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Re: [RFC v4 0/5] Add packed virtqueue to shadow virtqueue

From: Sahil Siddiq
Subject: Re: [RFC v4 0/5] Add packed virtqueue to shadow virtqueue
Date: Tue, 17 Dec 2024 11:15:03 +0530
User-agent: Mozilla Thunderbird 
Hi,

Thank you for your reply.

On 12/16/24 2:09 PM, Eugenio Perez Martin wrote:

On Sun, Dec 15, 2024 at 6:27 PM Sahil Siddiq <icegambit91@gmail.com> wrote:

On 12/10/24 2:57 PM, Eugenio Perez Martin wrote:

On Thu, Dec 5, 2024 at 9:34 PM Sahil Siddiq <icegambit91@gmail.com> wrote:

[...]
I have been following the "Hands on vDPA: what do you do
when you ain't got the hardware v2 (Part 2)" [1] blog to
test my changes. To boot the L1 VM, I ran:

sudo ./qemu/build/qemu-system-x86_64 \
-enable-kvm \
-drive 
file=//home/valdaarhun/valdaarhun/qcow2_img/L1.qcow2,media=disk,if=virtio \
-net nic,model=virtio \
-net user,hostfwd=tcp::2222-:22 \
-device intel-iommu,snoop-control=on \
-device 
virtio-net-pci,netdev=net0,disable-legacy=on,disable-modern=off,iommu_platform=on,guest_uso4=off,guest_uso6=off,host_uso=off,guest_announce=off,ctrl_vq=on,ctrl_rx=on,packed=on,event_idx=off,bus=pcie.0,addr=0x4
 \
-netdev tap,id=net0,script=no,downscript=no \
-nographic \
-m 8G \
-smp 4 \
-M q35 \
-cpu host 2>&1 | tee vm.log

Without "guest_uso4=off,guest_uso6=off,host_uso=off,
guest_announce=off" in "-device virtio-net-pci", QEMU
throws "vdpa svq does not work with features" [2] when
trying to boot L2.

The enums added in commit #2 in this series is new and
wasn't in the earlier versions of the series. Without
this change, x-svq=true throws "SVQ invalid device feature
flags" [3] and x-svq is consequently disabled.

The first issue is related to running traffic in L2
with vhost-vdpa.

In L0:

$ ip addr add 111.1.1.1/24 dev tap0
$ ip link set tap0 up
$ ip addr show tap0
4: tap0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state 
UNKNOWN group default qlen 1000
      link/ether d2:6d:b9:61:e1:9a brd ff:ff:ff:ff:ff:ff
      inet 111.1.1.1/24 scope global tap0
         valid_lft forever preferred_lft forever
      inet6 fe80::d06d:b9ff:fe61:e19a/64 scope link proto kernel_ll
         valid_lft forever preferred_lft forever

I am able to run traffic in L2 when booting without
x-svq.

In L1:

$ ./qemu/build/qemu-system-x86_64 \
-nographic \
-m 4G \
-enable-kvm \
-M q35 \
-drive file=//root/L2.qcow2,media=disk,if=virtio \
-netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa0 \
-device 
virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,bus=pcie.0,addr=0x7
 \
-smp 4 \
-cpu host \
2>&1 | tee vm.log

In L2:

# ip addr add 111.1.1.2/24 dev eth0
# ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP 
group default qlen 1000
      link/ether 52:54:00:12:34:57 brd ff:ff:ff:ff:ff:ff
      altname enp0s7
      inet 111.1.1.2/24 scope global eth0
         valid_lft forever preferred_lft forever
      inet6 fe80::9877:de30:5f17:35f9/64 scope link noprefixroute
         valid_lft forever preferred_lft forever

# ip route
111.1.1.0/24 dev eth0 proto kernel scope link src 111.1.1.2

# ping 111.1.1.1 -w3
PING 111.1.1.1 (111.1.1.1) 56(84) bytes of data.
64 bytes from 111.1.1.1: icmp_seq=1 ttl=64 time=0.407 ms
64 bytes from 111.1.1.1: icmp_seq=2 ttl=64 time=0.671 ms
64 bytes from 111.1.1.1: icmp_seq=3 ttl=64 time=0.291 ms

--- 111.1.1.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2034ms
rtt min/avg/max/mdev = 0.291/0.456/0.671/0.159 ms


But if I boot L2 with x-svq=true as shown below, I am unable
to ping the host machine.

$ ./qemu/build/qemu-system-x86_64 \
-nographic \
-m 4G \
-enable-kvm \
-M q35 \
-drive file=//root/L2.qcow2,media=disk,if=virtio \
-netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,x-svq=true,id=vhost-vdpa0 \
-device 
virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,bus=pcie.0,addr=0x7
 \
-smp 4 \
-cpu host \
2>&1 | tee vm.log

In L2:

# ip addr add 111.1.1.2/24 dev eth0
# ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP 
group default qlen 1000
      link/ether 52:54:00:12:34:57 brd ff:ff:ff:ff:ff:ff
      altname enp0s7
      inet 111.1.1.2/24 scope global eth0
         valid_lft forever preferred_lft forever
      inet6 fe80::9877:de30:5f17:35f9/64 scope link noprefixroute
         valid_lft forever preferred_lft forever

# ip route
111.1.1.0/24 dev eth0 proto kernel scope link src 111.1.1.2

# ping 111.1.1.1 -w10
PING 111.1.1.1 (111.1.1.1) 56(84) bytes of data.
  From 111.1.1.2 icmp_seq=1 Destination Host Unreachable
ping: sendmsg: No route to host
  From 111.1.1.2 icmp_seq=2 Destination Host Unreachable
  From 111.1.1.2 icmp_seq=3 Destination Host Unreachable

--- 111.1.1.1 ping statistics ---
3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2076ms
pipe 3

The other issue is related to booting L2 with "x-svq=true"
and "packed=on".

In L1:

$ ./qemu/build/qemu-system-x86_64 \
-nographic \
-m 4G \
-enable-kvm \
-M q35 \
-drive file=//root/L2.qcow2,media=disk,if=virtio \
-netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa0,x-svq=true \
-device 
virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,guest_uso4=off,guest_uso6=off,host_uso=off,guest_announce=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,packed=on,bus=pcie.0,addr=0x7
 \
-smp 4 \
-cpu host \
2>&1 | tee vm.log

The kernel throws "virtio_net virtio1: output.0:id 0 is not
a head!" [4].



So this series implements the descriptor forwarding from the guest to
the device in packed vq. We also need to forward the descriptors from
the device to the guest. The device writes them in the SVQ ring.

The functions responsible for that in QEMU are
hw/virtio/vhost-shadow-virtqueue.c:vhost_svq_flush, which is called by
the device when used descriptors are written to the SVQ, which calls
hw/virtio/vhost-shadow-virtqueue.c:vhost_svq_get_buf. We need to do
modifications similar to vhost_svq_add: Make them conditional if we're
in split or packed vq, and "copy" the code from Linux's
drivers/virtio/virtio_ring.c:virtqueue_get_buf.

After these modifications you should be able to ping and forward
traffic. As always, It is totally ok if it needs more than one
iteration, and feel free to ask any question you have :).



I misunderstood this part. While working on extending
hw/virtio/vhost-shadow-virtqueue.c:vhost_svq_get_buf() [1]
for packed vqs, I realized that this function and
vhost_svq_flush() already support split vqs. However, I am
unable to ping L0 when booting L2 with "x-svq=true" and
"packed=off" or when the "packed" option is not specified
in QEMU's command line.

I tried debugging these functions for split vqs after running
the following QEMU commands while following the blog [2].

Booting L1:

$ sudo ./qemu/build/qemu-system-x86_64 \
-enable-kvm \
-drive 
file=//home/valdaarhun/valdaarhun/qcow2_img/L1.qcow2,media=disk,if=virtio \
-net nic,model=virtio \
-net user,hostfwd=tcp::2222-:22 \
-device intel-iommu,snoop-control=on \
-device 
virtio-net-pci,netdev=net0,disable-legacy=on,disable-modern=off,iommu_platform=on,guest_uso4=off,guest_uso6=off,host_uso=off,guest_announce=off,ctrl_vq=on,ctrl_rx=on,packed=off,event_idx=off,bus=pcie.0,addr=0x4
 \
-netdev tap,id=net0,script=no,downscript=no \
-nographic \
-m 8G \
-smp 4 \
-M q35 \
-cpu host 2>&1 | tee vm.log

Booting L2:

# ./qemu/build/qemu-system-x86_64 \
-nographic \
-m 4G \
-enable-kvm \
-M q35 \
-drive file=//root/L2.qcow2,media=disk,if=virtio \
-netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,x-svq=true,id=vhost-vdpa0 \
-device 
virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,bus=pcie.0,addr=0x7
 \
-smp 4 \
-cpu host \
2>&1 | tee vm.log

I printed out the contents of VirtQueueElement returned
by vhost_svq_get_buf() in vhost_svq_flush() [3].
I noticed that "len" which is set by "vhost_svq_get_buf"
is always set to 0 while VirtQueueElement.len is non-zero.
I haven't understood the difference between these two "len"s.



VirtQueueElement.len is the length of the buffer, while the len of
vhost_svq_get_buf is the bytes written by the device. In the case of
the tx queue, VirtQueuelen is the length of the tx packet, and the
vhost_svq_get_buf is always 0 as the device does not write. In the
case of rx, VirtQueueElem.len is the available length for a rx frame,
and the vhost_svq_get_buf len is the actual length written by the
device.

To be 100% accurate a rx packet can span over multiple buffers, but
SVQ does not need special code to handle this.

So vhost_svq_get_buf should return > 0 for rx queue (svq->vq->index ==
0), and 0 for tx queue (svq->vq->index % 2 == 1).

Take into account that vhost_svq_get_buf only handles split vq at the
moment! It should be renamed or splitted into vhost_svq_get_buf_split.


In L1, there are 2 virtio network devices.

# lspci -nn | grep -i net
00:02.0 Ethernet controller [0200]: Red Hat, Inc. Virtio network device 
[1af4:1000]
00:04.0 Ethernet controller [0200]: Red Hat, Inc. Virtio 1.0 network device 
[1af4:1041] (rev 01)

I am using the second one (1af4:1041) for testing my changes and have
bound this device to the vp_vdpa driver.

# vdpa dev show -jp
{
    "dev": {
        "vdpa0": {
            "type": "network",
            "mgmtdev": "pci/0000:00:04.0",
            "vendor_id": 6900,
            "max_vqs": 3,
            "max_vq_size": 256
        }
    }
}

The max number of vqs is 3 with the max size being 256.

Since, there are 2 virtio net devices, vhost_vdpa_svqs_start [1]
is called twice. For each of them. it calls vhost_svq_start [2]
v->shadow_vqs->len number of times.

Printing the values of dev->vdev->name, v->shadow_vqs->len and
svq->vring.num in vhost_vdpa_svqs_start gives:

name: virtio-net
len: 2
num: 256
num: 256
name: virtio-net
len: 1
num: 64

I am not sure how to match the above log lines to the
right virtio-net device since the actual value of num
can be less than "max_vq_size" in the output of "vdpa
dev show".

I think the first 3 log lines correspond to the virtio
net device that I am using for testing since it has
2 vqs (rx and tx) while the other virtio-net device
only has one vq.

When printing out the values of svq->vring.num,
used_elem.len and used_elem.id in vhost_svq_get_buf,
there are two sets of output. One set corresponds to
svq->vring.num = 64 and the other corresponds to
svq->vring.num = 256.

For svq->vring.num = 64, only the following line
is printed repeatedly:

size: 64, len: 1, i: 0

For svq->vring.num = 256, the following line is
printed 20 times,

size: 256, len: 0, i: 0

followed by:

size: 256, len: 0, i: 1
size: 256, len: 0, i: 1

used_elem.len is used to set the value of len that is
returned by vhost_svq_get_buf, and it's always 0.

So the value of "len" returned by vhost_svq_get_buf
when called in vhost_svq_flush is also 0.

Thanks,
Sahil

[1] 
https://gitlab.com/qemu-project/qemu/-/blob/master/hw/virtio/vhost-vdpa.c#L1243
[2] 
https://gitlab.com/qemu-project/qemu/-/blob/master/hw/virtio/vhost-vdpa.c#L1265
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