Re: [PATCH v2 3/3] vhost: add device op to offload the interrupt kick

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 5/16/23 13:36, Eelco Chaudron wrote:
> 
> 
> On 16 May 2023, at 12:12, David Marchand wrote:
> 
>> On Tue, May 16, 2023 at 10:53 AM Eelco Chaudron <echaudro@redhat.com> wrote:
>>> On 10 May 2023, at 13:44, David Marchand wrote:
>>
>> [snip]
>>
>>>>> @@ -846,6 +848,14 @@ vhost_user_socket_mem_free(struct vhost_user_socket *vsocket)
>>>>>                  vsocket->path = NULL;
>>>>>          }
>>>>>
>>>>> +       if (vsocket && vsocket->alloc_notify_ops) {
>>>>> +#pragma GCC diagnostic push
>>>>> +#pragma GCC diagnostic ignored "-Wcast-qual"
>>>>> +               free((struct rte_vhost_device_ops *)vsocket->notify_ops);
>>>>> +#pragma GCC diagnostic pop
>>>>> +               vsocket->notify_ops = NULL;
>>>>> +       }
>>>>
>>>> Rather than select the behavior based on a boolean (and here force the
>>>> compiler to close its eyes), I would instead add a non const pointer
>>>> to ops (let's say alloc_notify_ops) in vhost_user_socket.
>>>> The code can then unconditionnally call free(vsocket->alloc_notify_ops);
>>>
>>> Good idea, I will make the change in v3.
>>
>> Feel free to use a better name for this field :-).
>>
>>>
>>>>> +
>>>>>          if (vsocket) {
>>>>>                  free(vsocket);
>>>>>                  vsocket = NULL;
>>
>> [snip]
>>
>>>>> +       /*
>>>>> +        * Although the ops structure is a const structure, we do need to
>>>>> +        * override the guest_notify operation. This is because with the
>>>>> +        * previous APIs it was "reserved" and if any garbage value was passed,
>>>>> +        * it could crash the application.
>>>>> +        */
>>>>> +       if (ops && !ops->guest_notify) {
>>>>
>>>> Hum, as described in the comment above, I don't think we should look
>>>> at ops->guest_notify value at all.
>>>> Checking ops != NULL should be enough.
>>>
>>> Not sure I get you here. If the guest_notify passed by the user is NULL, it means the previously ‘reserved[1]’ field is NULL, so we do not need to use a new structure.
>>>
>>> I guess your comment would be true if we would introduce a new field in the data structure, not replacing a reserved one.
>>
>> Hum, I don't understand my comment either o_O'.
>> Too many days off... or maybe my evil twin took over the keyboard.
>>
>>
>>>
>>>>> +               struct rte_vhost_device_ops *new_ops;
>>>>> +
>>>>> +               new_ops = malloc(sizeof(*new_ops));
>>>>
>>>> Strictly speaking, we lose the numa affinity of "ops" by calling malloc.
>>>> I am unclear of the impact though.
>>>
>>> Don’t think there is a portable API that we can use to determine the NUMA for the ops memory and then allocate this on the same numa?
>>>
>>> Any thoughts or ideas on how to solve this? I hope most people will memset() the ops structure and the reserved[1] part is zero, but it might be a problem in the future if more extensions get added.
>>
>> Determinining current numa is doable, via 'ops'
>> get_mempolicy(MPOL_F_NODE | MPOL_F_ADDR), like what is done for vq in
>> numa_realloc().
>> The problem is how to allocate on this numa with the libc allocator
>> for which I have no idea...
>> We could go with the dpdk allocator (again, like numa_realloc()).
>>
>>
>> In practice, the passed ops will be probably from a const variable in
>> the program .data section (for which I think fields are set to 0
>> unless explicitly initialised), or a memset() will be called for a
>> dynamic allocation from good citizens.
>> So we can probably live with the current proposal.
>> Plus, this is only for one release, since in 23.11 with the ABI bump,
>> we will drop this compat code.
>>
>> Maxime, Chenbo, what do you think?
> 
> Wait for their response, but for now I assume we can just keep the numa unaware malloc().

Let's keep it as is as we'll get rid of it in 23.11.

>>
>> [snip]
>>
>>>>
>>>> But putting indentation aside, is this change equivalent?
>>>> -               if ((vhost_need_event(vhost_used_event(vq), new, old) &&
>>>> -                                       (vq->callfd >= 0)) ||
>>>> -                               unlikely(!signalled_used_valid)) {
>>>> +               if ((vhost_need_event(vhost_used_event(vq), new, old) ||
>>>> +                               unlikely(!signalled_used_valid)) &&
>>>> +                               vq->callfd >= 0) {
>>>
>>> They are not equal, but in the past eventfd_write() should also not have been called with callfd < 0, guess this was an existing bug ;)
>>
>> I think this should be a separate fix.
> 
> ACK, will add a separate patch in this series to fix it.

I also caught & fixed it while implementing my VDUSE series [0].
You can pick it in your series, and I will rebase my series on top of
it.

Thanks,
Maxime

[0]: 
https://gitlab.com/mcoquelin/dpdk-next-virtio/-/commit/b976e1f226db5c09834148847d994045eb89be93


> 
>>
>>>
>>>>> +                       vhost_vring_inject_irq(dev, vq);
>>
>>
>> -- 
>> David Marchand
>