Hi Sasha,
On Thu, Oct 25, 2018 at 10:10:40AM -0400, Sasha Levin wrote:
From: Paul Burton paul.burton@mips.com
[ Upstream commit 906d441febc0de974b2a6ef848a8f058f3bfada3 ]
Some versions of GCC for the MIPS architecture suffer from a bug which can lead to instructions from beyond an unreachable statement being incorrectly reordered into earlier branch delay slots if the unreachable statement is the only content of a case in a switch statement. This can lead to seemingly random behaviour, such as invalid memory accesses from incorrectly reordered loads or stores, and link failures on microMIPS builds.
See this potential GCC fix for details:
https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.htmlRuntime problems resulting from this bug were initially observed using a maltasmvp_defconfig v4.4 kernel built using GCC 4.9.2 (from a Codescape SDK 2015.06-05 toolchain), with the result being an address exception taken after log messages about the L1 caches (during probe of the L2 cache):
Initmem setup node 0 [mem 0x0000000080000000-0x000000009fffffff] VPE topology {2,2} total 4 Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes. Primary data cache 64kB, 4-way, PIPT, no aliases, linesize 32 bytes <AdEL exception here>This is early enough that the kernel exception vectors are not in use, so any further output depends upon the bootloader. This is reproducible in QEMU where no further output occurs - ie. the system hangs here. Given the nature of the bug it may potentially be hit with differing symptoms. The bug is known to affect GCC versions as recent as 7.3, and it is unclear whether GCC 8 fixed it or just happens not to encounter the bug in the testcase found at the link above due to differing optimizations.
This bug can be worked around by placing a volatile asm statement, which GCC is prevented from reordering past, prior to the __builtin_unreachable call.
That was actually done already for other reasons by commit 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()"), but creates problems for microMIPS builds due to the lack of a .insn directive. The microMIPS ISA allows for interlinking with regular MIPS32 code by repurposing bit 0 of the program counter as an ISA mode bit. To switch modes one changes the value of this bit in the PC. However typical branch instructions encode their offsets as multiples of 2-byte instruction halfwords, which means they cannot change ISA mode - this must be done using either an indirect branch (a jump-register in MIPS terminology) or a dedicated jalx instruction. In order to ensure that regular branches don't attempt to target code in a different ISA which they can't actually switch to, the linker will check that branch targets are code in the same ISA as the branch.
Unfortunately our empty asm volatile statements don't qualify as code, and the link for microMIPS builds fails with errors such as:
arch/mips/mm/dma-default.s:3265: Error: branch to a symbol in another ISA mode arch/mips/mm/dma-default.s:5027: Error: branch to a symbol in another ISA modeResolve this by adding a .insn directive within the asm statement which declares that what comes next is code. This may or may not be true, since we don't really know what comes next, but as this code is in an unreachable path anyway that doesn't matter since we won't execute it.
We do this in asm/compiler.h & select CONFIG_HAVE_ARCH_COMPILER_H in order to have this included by linux/compiler_types.h after linux/compiler-gcc.h. This will result in asm/compiler.h being included in all C compilations via the -include linux/compiler_types.h argument in c_flags, which should be harmless.
Signed-off-by: Paul Burton paul.burton@mips.com Fixes: 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()") Patchwork: https://patchwork.linux-mips.org/patch/20270/ Cc: James Hogan jhogan@kernel.org Cc: Ralf Baechle ralf@linux-mips.org Cc: Arnd Bergmann arnd@arndb.de Cc: linux-mips@linux-mips.org Signed-off-by: Sasha Levin sashal@kernel.org
arch/mips/Kconfig | 1 + arch/mips/include/asm/compiler.h | 35 ++++++++++++++++++++++++++++++++ 2 files changed, 36 insertions(+)
In principle I'm fine with backporting this - it does fix broken builds.
It's only going to be of any use though if you also backport commit 04f264d3a8b0 ("compiler.h: Allow arch-specific asm/compiler.h"). I'd recommend backporting both or neither.
In practice I think it's unlikely anyone will need a microMIPS kernel & be tied to the particular versions affected by the bug this patch fixed, so I don't think it's a problem to backport neither.
Thanks, Paul