On Wed, Sep 02, 2020 at 03:58:38PM +0000, David Laight wrote: > From: Arvind Sankar > > Sent: 02 September 2020 16:34 > > > > The CRn accessor functions use __force_order as a dummy operand to > > prevent the compiler from reordering the inline asm. > > > > The fact that the asm is volatile should be enough to prevent this > > already, however older versions of GCC had a bug that could sometimes > > result in reordering. This was fixed in 8.1, 7.3 and 6.5. Versions prior > > to these, including 5.x and 4.9.x, may reorder volatile asm. > > > > There are some issues with __force_order as implemented: > > - It is used only as an input operand for the write functions, and hence > > doesn't do anything additional to prevent reordering writes. > > - It allows memory accesses to be cached/reordered across write > > functions, but CRn writes affect the semantics of memory accesses, so > > this could be dangerous. > > - __force_order is not actually defined in the kernel proper, but the > > LLVM toolchain can in some cases require a definition: LLVM (as well > > as GCC 4.9) requires it for PIE code, which is why the compressed > > kernel has a definition, but also the clang integrated assembler may > > consider the address of __force_order to be significant, resulting in > > a reference that requires a definition. > > > > Fix this by: > > - Using a memory clobber for the write functions to additionally prevent > > caching/reordering memory accesses across CRn writes. > > - Using a dummy input operand with an arbitrary constant address for the > > read functions, instead of a global variable. This will prevent reads > > from being reordered across writes, while allowing memory loads to be > > cached/reordered across CRn reads, which should be safe. > > How much does using a full memory clobber for the reads cost? > > It would remove any chance that the compiler decides it needs to > get the address of the 'dummy' location into a register so that > it can be used as a memory reference in a generated instruction > (which is probably what was happening for PIE compiles). > > David >
It doesn't cost much. When I tested it, the only differences were in startup code and sleep/hibernate/reboot code. The compiler doesn't load 0x1000 into a register even for PIE code, the reason it was doing it with a real symbol is to go through the GOT. Thanks.
