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1KASAN is supported on powerpc on 32-bit and Radix 64-bit only.2 332 bit support4==============5 6KASAN is supported on both hash and nohash MMUs on 32-bit.7 8The shadow area sits at the top of the kernel virtual memory space above the9fixmap area and occupies one eighth of the total kernel virtual memory space.10 11Instrumentation of the vmalloc area is optional, unless built with modules,12in which case it is required.13 1464 bit support15==============16 17Currently, only the radix MMU is supported. There have been versions for hash18and Book3E processors floating around on the mailing list, but nothing has been19merged.20 21KASAN support on Book3S is a bit tricky to get right:22 23 - It would be good to support inline instrumentation so as to be able to catch24 stack issues that cannot be caught with outline mode.25 26 - Inline instrumentation requires a fixed offset.27 28 - Book3S runs code with translations off ("real mode") during boot, including a29 lot of generic device-tree parsing code which is used to determine MMU30 features.31 32 - Some code - most notably a lot of KVM code - also runs with translations off33 after boot.34 35 - Therefore any offset has to point to memory that is valid with36 translations on or off.37 38One approach is just to give up on inline instrumentation. This way boot-time39checks can be delayed until after the MMU is set is up, and we can just not40instrument any code that runs with translations off after booting. This is the41current approach.42 43To avoid this limitation, the KASAN shadow would have to be placed inside the44linear mapping, using the same high-bits trick we use for the rest of the linear45mapping. This is tricky:46 47 - We'd like to place it near the start of physical memory. In theory we can do48 this at run-time based on how much physical memory we have, but this requires49 being able to arbitrarily relocate the kernel, which is basically the tricky50 part of KASLR. Not being game to implement both tricky things at once, this51 is hopefully something we can revisit once we get KASLR for Book3S.52 53 - Alternatively, we can place the shadow at the _end_ of memory, but this54 requires knowing how much contiguous physical memory a system has _at compile55 time_. This is a big hammer, and has some unfortunate consequences: inablity56 to handle discontiguous physical memory, total failure to boot on machines57 with less memory than specified, and that machines with more memory than58 specified can't use it. This was deemed unacceptable.59