RE: [PATCH v2] x86/asm: Replace __force_order with memory clobber

[David Laight]

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

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