Re: [dpdk-dev] [PATCH v2] net/virtio: add platform memory ordering feature support

[Ilya Maximets <i.maximets@samsung.com>]

On 15.01.2019 11:55, Shahaf Shuler wrote:
> Tuesday, January 15, 2019 10:29 AM, Ilya Maximets:
>> Subject: Re: [dpdk-dev] [PATCH v2] net/virtio: add platform memory
>> ordering feature support
>>
>> On 15.01.2019 9:33, Shahaf Shuler wrote:
>>> Thursday, January 10, 2019 10:37 PM, Shahaf Shuler:
>>>> Subject: RE: [dpdk-dev] [PATCH v2] net/virtio: add platform memory
>>>> ordering feature support
>>>>
>>>> Wednesday, January 9, 2019 5:50 PM, Michael S. Tsirkin:
>>>>> alejandro.lucero@netronome.com; Daniel Marcovitch
>>>>> <danielm@mellanox.com>
>>>>> Subject: Re: [dpdk-dev] [PATCH v2] net/virtio: add platform memory
>>>>> ordering feature support
>>>>>
>>>>> On Wed, Jan 09, 2019 at 05:34:38PM +0300, Ilya Maximets wrote:
>>>>>> virtio_mb() is really heavy. I'd like to avoid it somehow, but I
>>>>>> don't know how to do this yet.
>>>>>
>>>>> Linux driver doesn't avoid it either.
>>>>
>>>> I understand v3 was merged but still would like to continue the
>>>> discuss and make sure all is clear and agreed.
>>>>
>>>> Form patch [1] description it is very clear why we need the
>>>> rte_smp_mb() barrier.
>>>> However I am not sure why this barrier is interoperate into rte_mb in
>>>> case of vDPA.  In vDPA case, both read of the user ring and write of
>>>> the avail index are for local cached memory.
>>>> The only write which is to uncachable memory (device memory) is the
>>>> notify itself.
>>>>
>>>> As I mentioned, there is a need to have a store fence before doing
>>>> the notify, but from different reasons. So vDPA use case and need Is
>>>> a bit different than what presented in [1].
>>>
>>> Any answer?
>>> It is pity if we add redundant barriers which will degrade the driver
>> performance.
>>
>> Sorry for late responses. Have a lot of work with OVS right now.
>>
>> Regarding your question.
>> Current code looks like this:
>>
>>  1. Update ring.
>>  2. virtio_wmb()
>>  3. Update idx.
>>  4. virtio_mb()
>>  5. read flags.
>>  6. notify.
>>
>> virtio_mb() is here to avoid reordering of steps 3 and 5.
>> i.e. we need a full barrier to ensure the order between store (idx update)
>> and load (reading the flags). Otherwise we could miss the notification.
>> We can't avoid the barrier here, because even x86 does not guarantee the
>> ordering of the local load with earlier local store.
> 
> This is clear. You need the rte_smp_mb() here. My question is why you need the rte_mb() in case of vDPA? 
> As you said, all accesses are local.

Sorry, I misunderstood your question.
Memory accesses are not local here. We want the ring data/idx be visible
to vDPA HW before reading the flags.

> 
> Pasting you commit code:
> /*                                                                        
>  * Per virtio_ring.h in Linux.                                            
>  *     For virtio_pci on SMP, we don't need to order with respect to MMIO 
>  *     accesses through relaxed memory I/O windows, so smp_mb() et al are 
>  *     sufficient.                                                        
>  *                                                                        
>  *     For using virtio to talk to real devices (eg. vDPA) we do need real
>  *     barriers.                                                          
>  */                                                                       
> static inline void                                                        
> virtio_mb(uint8_t weak_barriers)                                          
> {                                                                         
>         if (weak_barriers)                                                
>                 rte_smp_mb();                                             
>         else                                                              
>                 rte_mb();                                                 
> }                                                                         
> 
>>
>>>
>>>>
>>>> [1]
>>>>
>> https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpa
>>>>
>> tches.dpdk.org%2Fpatch%2F49545%2F&amp;data=02%7C01%7Cshahafs%40
>> mellan
>>>>
>> ox.com%7C01907f1b2e0e4002cb7508d67ac38a98%7Ca652971c7d2e4d9ba6a4
>> d1492
>>>>
>> 56f461b%7C0%7C0%7C636831377591864200&amp;sdata=TSpc%2Fzyq2aq0N3
>> %2Bh4o
>>>> ro4std8ut%2FQU6%2BOeMDeeaQdsM%3D&amp;reserved=0
>>>>
>>>>>
>>>>> --
>>>>> MST
>>>
>>>