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

[Shahaf Shuler <shahafs@mellanox.com>]

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. 

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
> >
> >