Re: [Qemu-devel] vhost-user: VHOST_SET_MEM_TABLE, VHOST_SET_VRING_CALL need a reply?

[Linhaifeng]

On 2014/9/18 13:16, Michael S. Tsirkin wrote:
> On Thu, Sep 18, 2014 at 08:45:37AM +0800, Linhaifeng wrote:
>>
>>
>> On 2014/9/17 17:56, Michael S. Tsirkin wrote:
>>> On Wed, Sep 17, 2014 at 05:39:04PM +0800, Linhaifeng wrote:
>>>> I think maybe is not need for the backend to wait for response.
>>>>
>>>> There is another way.vhost-user send "VHOST_GET_MEM_TABLE" to qemu then 
>>>> qemu send VHOST_SET_MEM_TABLE to update the regions of vhost-user.same as 
>>>> other command.
>>>> If qemu could response the request of the vhost-user.the vhost-user could 
>>>> update date at anytime.
>>>
>>> The updates are initiated by QEMU, as a result of IOMMU,
>>> memory hotplug or some other configuration change.
>>
>> How to deal with the vhost-user restart?
>> when vhost-user restart it will lost the infomation which QEMU send.
>>
>> In the kernel mode vhost will restart with QEMU but in the user mode vhost 
>> will not.
> 
> vhost-user must restart with qemu only.
> 


Sometimes qemu not allowed to restart. e.g. The customer want to update the 
vhost-user to a newer version but don't want to restart the VM.


> The nature of virtio protocol is such that there's not enough in-memory
> state for host to gracefully recover from losing VQ state.
> 
> We could add a new feature to allow recovery by reporting
> failure to guest, and disabling processing new requests.
> Guest could respond by recovering / discarding submitted
> requests, re-enabling the device (likely by executing a reset)
> and re-submitting requests.
> 
> 
> This would need a new feature bit though, and would have to
> be acknowledged by guest. As such this would have to be
> virtio 1.0 feature, virtio 0.x is frozen.
> 
>>>
>>>> I think it's very useful for Commercialization.
>>>>
>>>> On 2014/9/17 16:38, Michael S. Tsirkin wrote:
>>>>> Reply-To: 
>>>>>
>>>>> Thinking about the vhost-user protocol, VHOST_SET_MEM_TABLE
>>>>> is used to update the memory mappings.
>>>>>
>>>>> So shouldn't we want for response?
>>>>> Otherwise e.g. guest can start using the memory
>>>>> that vhost-user can't access.
>>>>>
>>>>> Similarly, with an IOMMU vhost-user might access memory it shouldn't.
>>>>>
>>>>> VHOST_SET_VRING_CALL is used for MSI-X masking.
>>>>> Again, after vector is masted by switching the call fd,
>>>>> backend shouldn't assert the old one.
>>>>>
>>>>> Thoughts?
>>>>>
>>>>>
>>>
>>> .
>>>
> 
> .
>