Get a vm fd using kvm API's ioctls

Get a vm fd using kvm API's ioctls

[Vincent KHERBACHE]

Hi all,

I'm trying to get the dirty bitmap of a specific VM, using
KVM_GET_DIRTY_LOG ioctl.

For this purpose, I should be able to get the file descriptor of an
existing VM by doing something like :

kvm_fd = open("/dev/kvm")
...
b = ioctl(KVM_GET_DIRTY_LOG, vm_fd)


I also can see, from the API documentation
(https://www.kernel.org/doc/Documentation/virtual/kvm/api.txt), that
there is the following restriction concerning VM ioctls :

"Only run VM ioctls from the same process (address space) that was used
to create the VM."


Is there a way to get the fd of a running VM (created from an other
process), or maybe a better/easier manner to get the dirty bitmap ?


Any help would be welcome.
Thanks,

-- 
Vincent KHERBACHE

Re: Get a vm fd using kvm API's ioctls

[Paolo Bonzini]

Il 05/02/2014 17:30, Vincent KHERBACHE ha scritto:
> Hi all,
>
> I'm trying to get the dirty bitmap of a specific VM, using
> KVM_GET_DIRTY_LOG ioctl.
>
> For this purpose, I should be able to get the file descriptor of an
> existing VM by doing something like :
>
> kvm_fd = open("/dev/kvm")
> ...
> b = ioctl(KVM_GET_DIRTY_LOG, vm_fd)
>
>
> I also can see, from the API documentation
> (https://www.kernel.org/doc/Documentation/virtual/kvm/api.txt), that
> there is the following restriction concerning VM ioctls :
>
> "Only run VM ioctls from the same process (address space) that was used
> to create the VM."
>
>
> Is there a way to get the fd of a running VM (created from an other
> process), or maybe a better/easier manner to get the dirty bitmap ?

You can get ask the other process to retrieve the dirty bitmap and place 
it in a shared memory segment.

However, note that KVM_GET_DIRTY_LOG retrieves _and clears_ the dirty 
bitmap.  So if the "owner" of the running VM is already using the dirty 
bitmap, calling KVM_GET_DIRTY_LOG will likely break that usage.

Paolo

Re: Get a vm fd using kvm API's ioctls

[Vincent KHERBACHE]

Le 05/02/2014 17:52, Paolo Bonzini a écrit :
> Il 05/02/2014 17:30, Vincent KHERBACHE ha scritto:
>> Hi all,
>>
>> I'm trying to get the dirty bitmap of a specific VM, using
>> KVM_GET_DIRTY_LOG ioctl.
>>
>> For this purpose, I should be able to get the file descriptor of an
>> existing VM by doing something like :
>>
>> kvm_fd = open("/dev/kvm")
>> ...
>> b = ioctl(KVM_GET_DIRTY_LOG, vm_fd)
>>
>>
>> I also can see, from the API documentation
>> (https://www.kernel.org/doc/Documentation/virtual/kvm/api.txt), that
>> there is the following restriction concerning VM ioctls :
>>
>> "Only run VM ioctls from the same process (address space) that was used
>> to create the VM."
>>
>>
>> Is there a way to get the fd of a running VM (created from an other
>> process), or maybe a better/easier manner to get the dirty bitmap ?
> 
> You can get ask the other process to retrieve the dirty bitmap and place
> it in a shared memory segment.
> 
> However, note that KVM_GET_DIRTY_LOG retrieves _and clears_ the dirty
> bitmap.  So if the "owner" of the running VM is already using the dirty
> bitmap, calling KVM_GET_DIRTY_LOG will likely break that usage.


Thank you for your reply !

I take note of your remark about the bitmap clearing, it could be an
effective issue.

But I really have no idea of how can I 'ask' something the other process ?


Regards.
-- 
Vincent KHERBACHE

Re: Get a vm fd using kvm API's ioctls

[Paolo Bonzini]

Il 05/02/2014 18:17, Vincent KHERBACHE ha scritto:
> Thank you for your reply !
>
> I take note of your remark about the bitmap clearing, it could be an
> effective issue.
>
> But I really have no idea of how can I 'ask' something the other process ?

You modify the source code for that program. :)  It's likely QEMU or 
kvmtool, in either case it's free software. :)

Paolo

Re: Get a vm fd using kvm API's ioctls

[Vincent KHERBACHE]

Ok this is what I feared, modifying Qemu is the only way to get the dirty bitmap from a VM launched by a Qemu process..

Ans this is definitely what I will do. Thanks again for your help !


Paolo Bonzini <pbonzini@redhat.com> a écrit :
>Il 05/02/2014 18:17, Vincent KHERBACHE ha scritto:
>> Thank you for your reply !
>>
>> I take note of your remark about the bitmap clearing, it could be an
>> effective issue.
>>
>> But I really have no idea of how can I 'ask' something the other
>process ?
>
>You modify the source code for that program. :)  It's likely QEMU or 
>kvmtool, in either case it's free software. :)
>
>Paolo

-- 
Vincent KHERBACHE