Re: [PATCH] vhost: call write barrier before used index update

[Bruce Richardson <bruce.richardson@intel.com>]

On 21/10/2015 16:42, Nikita Kalyazin wrote:
> Hi,
>
>> This has been discussed a number of times before on list, and the consensus
>> seems to be that the correct way to fix this is to introduce a set of specific
>> barrier operations that insert the correct barrier type on each architecture,
>> i.e. compiler barriers on IA, and full wmbs on architectures that require that.
> Linux kernel contains two sets of macros: *mb() and smp_*mb().  As far as
> I understand, the former are meant to order memory accesses when interacting
> with peripherals (physical NICs in our case), and the latter - to provide
> synchronization between CPU cores (applicable for virtual NICs in our case).
> smp_*mb() for Intel architecture would be a simple compiler barrier, whereas
> for processors with weaker memory model they may use real barrier
> instructions.
> Maybe implementing barriers similar way would work in DPDK as well?
>
It should work indeed.
Not sure if we need two separate sets of barrier functions, the 
smb_*mb() might work on their own (certainly for IA, they should do - 
not sure for other architectures), but I think the general consensus, 
based on previous discussions, is that this is the direction we want to 
go in for these barrier issues.

/Bruce