Re: [dpdk-dev] [RFC] mempool: add non-IO flag

[Thomas Monjalon <thomas@monjalon.net>]

+1, I support this idea.

12/08/2021 14:43, Dmitry Kozlyuk:
> We propose to add a mempool flag MEMPOOL_F_NON_IO to mark pools of objects that
> will not be used with device IO and their memory for DMA.  This will allow
> saving IOMMU entries by not mapping the memory used by such pools.
> 
> Immediate use case is MLX5 PMD.  The hardware has its internal IOMMU where PMD
> registers the memory.  On the data path, PMD translates VA into a key consumed
> by the device IOMMU.  It is impractical for the PMD to register all allocated
> memory because of increased lookup cost both in HW and SW.  Most often mbuf
> memory comes from mempools, so if PMD tracked them, it could almost always have
> mbuf memory registered before an mbuf hits the PMD. The new flag would prevent
> the PMD for registering memory that will never need it. Tracking the mempools
> and dealing with them in MLX5 PMD is the next step after the proposed change.
> 
> A possible use case is IOMMU management in EAL.  Mempool could translate the new
> flag to a hint to the memory manager, which would use it to skip adding IOMMU
> entries in some cases.
> 
> It was considered to add MEMPOOL_F_IO with the opposite meaning. It would be
> automatically set for pktmbuf pools; user would be able to set it for other
> pools.  However, current assumption is that all DPDK memory is DMA-able,
> it is controversial to have a flag asserting this fact.