On 11/26/2020 9:09 AM, Ard Biesheuvel wrote: > On Wed, 25 Nov 2020 at 22:39, Iuliana Prodan <iuliana.prodan@xxxxxxx> wrote: >> >> On 11/25/2020 11:16 PM, Ard Biesheuvel wrote: >>> On Wed, 25 Nov 2020 at 22:14, Iuliana Prodan (OSS) >>> <iuliana.prodan@xxxxxxxxxxx> wrote: >>>> >>>> From: Iuliana Prodan <iuliana.prodan@xxxxxxx> >>>> >>>> Add the option to allocate the crypto request object plus any extra space >>>> needed by the driver into a DMA-able memory. >>>> >>>> Add CRYPTO_TFM_REQ_DMA flag to be used by backend implementations to >>>> indicate to crypto API the need to allocate GFP_DMA memory >>>> for private contexts of the crypto requests. >>>> >>> >>> These are always directional DMA mappings, right? So why can't we use >>> bounce buffering here? >>> >> The idea was to avoid allocating any memory in crypto drivers. >> We want to be able to use dm-crypt with CAAM, which needs DMA-able >> memory and increasing reqsize is not enough. > > But what does 'needs DMA-able memory' mean? DMA operations are > asynchronous by definition, and so the DMA layer should be able to > allocate bounce buffers when needed. This will cost some performance > in cases where the hardware cannot address all of memory directly, but > this is a consequence of the design, and I don't think we should > burden the generic API with this. > The performance loss due to bounce buffering is non-negligible. Previous experiments we did showed a 35% gain (when forcing all data, including I/O buffers, in ZONE_DMA32). I don't have the exact numbers for bounce buffering introduced by allowing only by the control data structures (descriptors etc.) in high memory, but I don't think it's fair to easily dismiss this topic, given the big performance drop and relatively low impact on the generic API. Thanks, Horia
