On Thu, 2016-08-04 at 18:19 +0200, Jesper Dangaard Brouer wrote: > I actually agree, that we should switch to order-0 allocations. > > *BUT* this will cause performance regressions on platforms with > expensive DMA operations (as they no longer amortize the cost of > mapping a larger page).
We much prefer reliable behavior, even it it is ~1 % slower than the super-optimized thing that opens highways for attackers. Anyway, in most cases pages are re-used, so we only call dma_sync_single_range_for_cpu(), and there is no way to avoid this. Using order-0 pages [1] is actually faster, since when we use high-order pages (multiple frames per 'page') we can not reuse the pages. [1] I had a local patch to allocate these pages using a very simple allocator allocating max order (order-10) pages and splitting them into order-0 ages, in order to lower TLB footprint. But I could not measure a gain doing so on x86, at least on my lab machines.
