On Friday 23 January 2015 14:52:36 Catalin Marinas wrote: > On Thu, Jan 22, 2015 at 11:03:00AM +0000, Arnd Bergmann wrote: > > Unrelated to this question however is whether we want to keep > > supersection mappings as a performance optimization to save TLBs. > > It seems useful to me, but not critical. > > Currently in Linux we allow 16MB mappings only if the phys address is > over 32-bit and !LPAE which makes it unlikely for normal RAM with > pre-LPAE hardware. Ah, I missed this part when looking at the code. > IIRC a bigger problem was that supersections are optional in the > architecture but there was no CPUID bit field in ARMv6 (and early ARMv7) > to check for their presence. The ID_MMFR3 contains this information but > for example on early Cortex-A8 that bitfield was reserved and the TRM > states "unpredictable" on read (so probably zero in practice). > > On newer ARMv7 (not necessarily with LPAE), we could indeed revisit the > 16MB section mapping but it won't go well with single zImage if you want > to support earlier ARMv7 or ARMv6. I see. If there is desire to have it as an optimization, we could do it for armv7ve-only kernels. We don't currently have an build-time option for those, but we should introduce one anyway, in order to better make use of the idiv instructions and to prevent one from enabling LPAE on a multiplatform kernel that contains pre-lpae armv7 machines (Cortex a8/a9/a5, and some others I'm not sure about). Arnd -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>