vsock support for communication between guests

vsock support for communication between guests

[Robert Hoo]

Hi,

Does vsock support communication between guests?
 From man page, and my experiment, seems it doesn't.
But why not?

Re: vsock support for communication between guests

[Stefano Garzarella]

On Fri, Oct 10, 2025 at 09:00:21PM +0800, Robert Hoo wrote:
>Hi,
>
>Does vsock support communication between guests?
>From man page, and my experiment, seems it doesn't.
>But why not?
>

It depends, vhost-user vsock device, supports it.
See 
https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-vsock#sibling-vm-communication

The vhost-vsock in-kernel device doesn't support it.

The main problem is that vsock is designed for host<->guest 
communication, so implementing a guest<->guest communication is 
possible, but requires more configuration (e.g. some kind of firewall, 
etc.) and also an extension to the address (see the required
`.svm_flags = VMADDR_FLAG_TO_HOST` in the link).

The easy way to do that with vhost-vsock, is to use socat in the host to 
concatenate 2 VMs (some examples here: 
https://stefano-garzarella.github.io/posts/2021-01-22-socat-vsock/)

Cheers,
Stefano

Re: vsock support for communication between guests

[Robert Hoo]

On 11/6/2025 10:32 PM, Stefano Garzarella wrote:
> On Fri, Oct 10, 2025 at 09:00:21PM +0800, Robert Hoo wrote:
>> Hi,
>>
>> Does vsock support communication between guests?
>> From man page, and my experiment, seems it doesn't.
>> But why not?
>>
> 
> It depends, vhost-user vsock device, supports it.
> See 
> https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-vsock#sibling-vm-communication
> 
> The vhost-vsock in-kernel device doesn't support it.
> 
> The main problem is that vsock is designed for host<->guest communication, so 
> implementing a guest<->guest communication is possible, but requires more 
> configuration (e.g. some kind of firewall, etc.) and also an extension to the 
> address (see the required
> `.svm_flags = VMADDR_FLAG_TO_HOST` in the link).
> 
> The easy way to do that with vhost-vsock, is to use socat in the host to 
> concatenate 2 VMs (some examples here: 
> https://stefano-garzarella.github.io/posts/2021-01-22-socat-vsock/)
> 
> Cheers,
> Stefano
> 
Nice, thanks Stefano. It sounds ideal for my VM <--> VM communication 
requirement. I'll read the doc carefully later.

BTW, I also found your vsock-bridge 
(https://github.com/stefano-garzarella/vsock-bridge); but seems its last commit 
was 5 yrs ago. It's not recommended, is it?

Re: vsock support for communication between guests

[Stefano Garzarella]

On Sun, 9 Nov 2025 at 14:13, Robert Hoo <robert.hoo.linux@gmail.com> wrote:
>
> On 11/6/2025 10:32 PM, Stefano Garzarella wrote:
> > On Fri, Oct 10, 2025 at 09:00:21PM +0800, Robert Hoo wrote:
> >> Hi,
> >>
> >> Does vsock support communication between guests?
> >> From man page, and my experiment, seems it doesn't.
> >> But why not?
> >>
> >
> > It depends, vhost-user vsock device, supports it.
> > See
> > https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-vsock#sibling-vm-communication
> >
> > The vhost-vsock in-kernel device doesn't support it.
> >
> > The main problem is that vsock is designed for host<->guest communication, so
> > implementing a guest<->guest communication is possible, but requires more
> > configuration (e.g. some kind of firewall, etc.) and also an extension to the
> > address (see the required
> > `.svm_flags = VMADDR_FLAG_TO_HOST` in the link).
> >
> > The easy way to do that with vhost-vsock, is to use socat in the host to
> > concatenate 2 VMs (some examples here:
> > https://stefano-garzarella.github.io/posts/2021-01-22-socat-vsock/)
> >
> > Cheers,
> > Stefano
> >
> Nice, thanks Stefano. It sounds ideal for my VM <--> VM communication
> requirement. I'll read the doc carefully later.
>
> BTW, I also found your vsock-bridge
> (https://github.com/stefano-garzarella/vsock-bridge); but seems its last commit
> was 5 yrs ago. It's not recommended, is it?
>

Oh, that was just a little exercise I did to learn Rust at the time,
so I'd say no, it's not recommended.
BTW `socat` supports a similar use case, so related to the example in
the vsock-bridge's README, you can do the following:

host$ socat VSOCK-LISTEN:5201 VSOCK-CONNECT:4:5201
vm_cid3$ iperf --vsock -s
vm_cid4$ iperf --vsock -c 2

But yeah, it's not 2 ways like vsock-bridge (i.e. `vm_cid3` can't
connect to `vm_cid4`).

Cheers,
Stefano

Re: vsock support for communication between guests

[Bobby Eshleman]

On Sun, Nov 09, 2025 at 09:13:13PM +0800, Robert Hoo wrote:
> On 11/6/2025 10:32 PM, Stefano Garzarella wrote:
> > On Fri, Oct 10, 2025 at 09:00:21PM +0800, Robert Hoo wrote:
> > > Hi,
> > > 
> > > Does vsock support communication between guests?
> > > From man page, and my experiment, seems it doesn't.
> > > But why not?
> > > 
> > 
> > It depends, vhost-user vsock device, supports it.
> > See https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-vsock#sibling-vm-communication
> > 
> > The vhost-vsock in-kernel device doesn't support it.
> > 
> > The main problem is that vsock is designed for host<->guest
> > communication, so implementing a guest<->guest communication is
> > possible, but requires more configuration (e.g. some kind of firewall,
> > etc.) and also an extension to the address (see the required
> > `.svm_flags = VMADDR_FLAG_TO_HOST` in the link).
> > 
> > The easy way to do that with vhost-vsock, is to use socat in the host to
> > concatenate 2 VMs (some examples here:
> > https://stefano-garzarella.github.io/posts/2021-01-22-socat-vsock/)
> > 
> > Cheers,
> > Stefano
> > 
> Nice, thanks Stefano. It sounds ideal for my VM <--> VM communication
> requirement. I'll read the doc carefully later.
> 

Though not as convenient as socat, VSOCK also supports sockmap which can
do this in-kernel (bpf), there are a few test cases for this in
selftests/bpf.

Best,
Bobby

Re: vsock support for communication between guests

[Robert Hoo]

On 11/10/2025 7:10 PM, Stefano Garzarella wrote:
> On Sun, 9 Nov 2025 at 14:13, Robert Hoo <robert.hoo.linux@gmail.com> wrote:
>>
>> On 11/6/2025 10:32 PM, Stefano Garzarella wrote:
>>> On Fri, Oct 10, 2025 at 09:00:21PM +0800, Robert Hoo wrote:
>>>> Hi,
>>>>
>>>> Does vsock support communication between guests?
>>>>  From man page, and my experiment, seems it doesn't.
>>>> But why not?
>>>>
>>>
>>> It depends, vhost-user vsock device, supports it.
>>> See
>>> https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-vsock#sibling-vm-communication
>>>
>>> The vhost-vsock in-kernel device doesn't support it.
>>>
>>> The main problem is that vsock is designed for host<->guest communication, so
>>> implementing a guest<->guest communication is possible, but requires more
>>> configuration (e.g. some kind of firewall, etc.) and also an extension to the
>>> address (see the required
>>> `.svm_flags = VMADDR_FLAG_TO_HOST` in the link).
>>>
>>> The easy way to do that with vhost-vsock, is to use socat in the host to
>>> concatenate 2 VMs (some examples here:
>>> https://stefano-garzarella.github.io/posts/2021-01-22-socat-vsock/)
>>>
>>> Cheers,
>>> Stefano
>>>
>> Nice, thanks Stefano. It sounds ideal for my VM <--> VM communication
>> requirement. I'll read the doc carefully later.
>>
>> BTW, I also found your vsock-bridge
>> (https://github.com/stefano-garzarella/vsock-bridge); but seems its last commit
>> was 5 yrs ago. It's not recommended, is it?
>>
> 
> Oh, that was just a little exercise I did to learn Rust at the time,
> so I'd say no, it's not recommended.
> BTW `socat` supports a similar use case, so related to the example in
> the vsock-bridge's README, you can do the following:
> 
> host$ socat VSOCK-LISTEN:5201 VSOCK-CONNECT:4:5201
> vm_cid3$ iperf --vsock -s
> vm_cid4$ iperf --vsock -c 2
> 
> But yeah, it's not 2 ways like vsock-bridge (i.e. `vm_cid3` can't
> connect to `vm_cid4`).
> 
> Cheers,
> Stefano
> 
Thanks Stefano.

Re: vsock support for communication between guests

[Robert Hoo]

On 11/11/2025 12:20 AM, Bobby Eshleman wrote:
> On Sun, Nov 09, 2025 at 09:13:13PM +0800, Robert Hoo wrote:
>> On 11/6/2025 10:32 PM, Stefano Garzarella wrote:
>>> On Fri, Oct 10, 2025 at 09:00:21PM +0800, Robert Hoo wrote:
>>>> Hi,
>>>>
>>>> Does vsock support communication between guests?
>>>>  From man page, and my experiment, seems it doesn't.
>>>> But why not?
>>>>
>>>
>>> It depends, vhost-user vsock device, supports it.
>>> See https://github.com/rust-vmm/vhost-device/tree/main/vhost-device-vsock#sibling-vm-communication
>>>
>>> The vhost-vsock in-kernel device doesn't support it.
>>>
>>> The main problem is that vsock is designed for host<->guest
>>> communication, so implementing a guest<->guest communication is
>>> possible, but requires more configuration (e.g. some kind of firewall,
>>> etc.) and also an extension to the address (see the required
>>> `.svm_flags = VMADDR_FLAG_TO_HOST` in the link).
>>>
>>> The easy way to do that with vhost-vsock, is to use socat in the host to
>>> concatenate 2 VMs (some examples here:
>>> https://stefano-garzarella.github.io/posts/2021-01-22-socat-vsock/)
>>>
>>> Cheers,
>>> Stefano
>>>
>> Nice, thanks Stefano. It sounds ideal for my VM <--> VM communication
>> requirement. I'll read the doc carefully later.
>>
> 
> Though not as convenient as socat, VSOCK also supports sockmap which can
> do this in-kernel (bpf), there are a few test cases for this in
> selftests/bpf.
> 
> Best,
> Bobby

Nice to know this, thanks Bobby. It a good idea. I'll look into the selftests 
for reference.