From: Alexander Duyck > Sent: 05 August 2016 16:15 ... > > > > interesting idea. Like dma_map 1GB region and then allocate > > pages from it only? but the rest of the kernel won't be able > > to use them? so only some smaller region then? or it will be > > a boot time flag to reserve this pseudo-huge page? > > Yeah, something like that. If we were already talking about > allocating a pool of pages it might make sense to just setup something > like this where you could reserve a 1GB region for a single 10G device > for instance. Then it would make the whole thing much easier to deal > with since you would have a block of memory that should perform very > well in terms of DMA accesses. ISTM that the main kernel allocator ought to be keeping a cache of pages that are mapped into the various IOMMU. This might be a per-driver cache, but could be much wider. Then if some code wants such a page it can be allocated one that is already mapped. Under memory pressure the pages could then be reused for other purposes. ... > In the Intel drivers for instance if the frame > size is less than 256 bytes we just copy the whole thing out since it > is cheaper to just extend the header copy rather than taking the extra > hit for get_page/put_page. How fast is 'rep movsb' (on cached addresses) on recent x86 cpu? It might actually be worth unconditionally copying the entire frame on those cpus. A long time ago we found the breakeven point for the copy to be about 1kb on sparc mbus/sbus systems - and that might not have been aligning the copy. David ��.n������g��� �a��� �&ޖ)���)��h���&������梷���� �Ǟ�m������)������^�����������v���O��zf������