Using signals to communicate two Qemu processes

Using signals to communicate two Qemu processes

[Passera, Pablo R]

Hi all,
        I am trying to communicate two VMs using a virtio driver. Once a data is moved to the driver I want to notify the other Qemu process that there is new data available in the buffer. I was thinking about using linux signals to synchronize both processes but when I register my SIGUSR1 handler in Qemu I am seeing an strange behavior. After starting the VM and Linux gets loaded, Qemu is receiving SIGUSR2 at a regular time period. Looking a little bit at the code I realize that signals are being used for other purposes in Qemu, however, SIGUSR1 is not used. Is it possible to use signals to synchronize these processes or should I think about using a different mechanism?

Thanks,

Pablo Pássera
Intel - Software Innovation Pathfinding Group
Cordoba - Argentina
Phone: +54 351 526 5611

Re: Using signals to communicate two Qemu processes

[Dor Laor]

Passera, Pablo R wrote:
> Hi all,
>         I am trying to communicate two VMs using a virtio driver. Once a data is moved to the driver I want to notify the other Qemu process that there is new data available in the buffer. I was thinking about using linux signals to synchronize both processes but when I register my SIGUSR1 handler in Qemu I am seeing an strange behavior. After starting the VM and Linux gets loaded, Qemu is receiving SIGUSR2 at a regular time period. Looking a little bit at the code I realize that signals are being used for other purposes in Qemu, however, SIGUSR1 is not used. Is it possible to use signals to synchronize these processes or should I think about using a different mechanism?
>
>   
SIGUSR2 is used as aio completion signal. You can use SIGUSR1 but you 
need to know what you're doing (some threads block signals).
Better fit would be a pipe.

The vcpu
> Thanks,
>
> Pablo Pássera
> Intel - Software Innovation Pathfinding Group
> Cordoba - Argentina
> Phone: +54 351 526 5611
>
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Re: Using signals to communicate two Qemu processes

[Anthony Liguori]

Dor Laor wrote:
> Passera, Pablo R wrote:
>> Hi all,
>>         I am trying to communicate two VMs using a virtio driver. 
>> Once a data is moved to the driver I want to notify the other Qemu 
>> process that there is new data available in the buffer. I was 
>> thinking about using linux signals to synchronize both processes but 
>> when I register my SIGUSR1 handler in Qemu I am seeing an strange 
>> behavior. After starting the VM and Linux gets loaded, Qemu is 
>> receiving SIGUSR2 at a regular time period. Looking a little bit at 
>> the code I realize that signals are being used for other purposes in 
>> Qemu, however, SIGUSR1 is not used. Is it possible to use signals to 
>> synchronize these processes or should I think about using a different 
>> mechanism?
>>
>>   
> SIGUSR2 is used as aio completion signal. You can use SIGUSR1 but you 
> need to know what you're doing (some threads block signals).
> Better fit would be a pipe.

Or even a unix domain socket.

Regards,

Anthony Liguori

Re: Using signals to communicate two Qemu processes

[Avi Kivity]

Passera, Pablo R wrote:
> Hi all,
>         I am trying to communicate two VMs using a virtio driver. Once a data is moved to the driver I want to notify the other Qemu process that there is new data available in the buffer. I was thinking about using linux signals to synchronize both processes but when I register my SIGUSR1 handler in Qemu I am seeing an strange behavior. After starting the VM and Linux gets loaded, Qemu is receiving SIGUSR2 at a regular time period. Looking a little bit at the code I realize that signals are being used for other purposes in Qemu, however, SIGUSR1 is not used. Is it possible to use signals to synchronize these processes or should I think about using a different mechanism?
>   

Signals are messy.  I recommend using a unix-domain socket for this.

It should be possible to use signals, however you should take care to 
mask the signal in all threads except the once you want to be the handler.

-- 
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