On Thu, Jan 7, 2016 at 2:16 AM, Chris Wilson chris@chris-wilson.co.uk wrote:
On Mon, Oct 19, 2015 at 10:58:55AM +0100, Chris Wilson wrote:
During testing we observed that the last cacheline was not being flushed from a
mb() for (addr = addr & -clflush_size; addr < end; addr += clflush_size) clflushopt(); mb()loop (where the initial addr and end were not cacheline aligned).
Changing the loop from addr < end to addr <= end, or replacing the clflushopt() with clflush() both fixed the testcase. Hinting that GCC was miscompling the assembly within the loop and specifically the alternative within clflushopt() was confusing the loop optimizer.
Adding a barrier() into clflushopt() is enough for GCC to dtrt, but solving why GCC is not seeing the constraints from the alternative_io() would be smarter...
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92501 Testcase: gem_tiled_partial_pwrite_pread/read Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Ross Zwisler ross.zwisler@linux.intel.com Cc: H. Peter Anvin hpa@linux.intel.com Cc: Imre Deak imre.deak@intel.com Cc: Daniel Vetter daniel.vetter@ffwll.ch Cc: dri-devel@lists.freedesktop.org
arch/x86/include/asm/special_insns.h | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h index 2270e41b32fd..0c7aedbf8930 100644 --- a/arch/x86/include/asm/special_insns.h +++ b/arch/x86/include/asm/special_insns.h @@ -199,6 +199,11 @@ static inline void clflushopt(volatile void *__p) ".byte 0x66; clflush %P0", X86_FEATURE_CLFLUSHOPT, "+m" (*(volatile char __force *)__p));
/* GCC (4.9.1 and 5.2.1 at least) appears to be very confused when* meeting this alternative() and demonstrably miscompiles loops* iterating over clflushopts.*/barrier();}
Or an alternative:
+#define alternative_output(oldinstr, newinstr, feature, output) \
asm volatile (ALTERNATIVE(oldinstr, newinstr, feature) \: output : "i" (0) : "memory")I would really appreciate some knowledgeable folks taking a look at the asm for clflushopt() as it still affects today's kernel and gcc.
Fwiw, I have confirmed that arch/x86/mm/pageattr.c clflush_cache_range() is similarly affected.
Unless I'm mis-reading the asm, clflush_cache_range() is compiled correctly for me. (I don't know what the %P is for in the asm, but that shouldn't matter.) The ALTERNATIVE shouldn't even be visible to the optimizer.
Can you attach a bad .s file and let us know what gcc version this is? (You can usually do 'make foo/bar/baz.s' to get a .s file.) I'd also be curious whether changing clflushopt to clwb works around the issue.
--Andy