On 10/14/2016 02:17 PM, York Sun wrote:
Current code turns off d-cache first, then flush all levels of cache.
This results data loss. As soon as d-cache is off, the dirty cache
is discarded according to the test on LS2080A. This issue was not
seen as long as external L3 cache was flushed to push the data to
main memory. However, external L3 cache is not guaranteed to have
the data. To fix this, flush the d-cache by way/set first to make
sure cache is clean before turning it off.
diff --git a/arch/arm/cpu/armv8/cache_v8.c b/arch/arm/cpu/armv8/cache_v8.c
@@ -478,9 +478,9 @@ void dcache_disable(void)
+ flush_dcache_all();
set_sctlr(sctlr & ~(CR_C|CR_M));
- flush_dcache_all();
__asm_invalidate_tlb_all();
I talked to Mark Rutland at ARM, and I believe the current code is
correct. Here's my interpretation of what he said:
The dcache must be disabled first. This prevents allocation of new
entries in the cache during the flush operation, which prevents the race
conditions that were mentioned in the other thread.
Then, the flush operation must be invoked. Since the cache is now
disabled, this can fully flush the cache without worrying about racing
with things being added to the cache.
This all implies that the implementation of dcache_disable(),
set_sctlr(), flush_dcache_all(), and any code they call must not access
data in DRAM at all; since because the dcache is off, any DRAM access
will[1] read potentially stale data from DRAM, rather than any dirty
data that might be in the cache.
[1] I'm not sure if that's "will" or "may", i.e. whether this is
architecturally guaranteed in ARMv8 or is implementation defined. At
least the Cortex A72 TRM says "will" for that CPU; not sure about others.
Perhaps the most obvious upshot of this is that the stack can't be used.
This implies to me that we need to recode all those functions purely in
assembly, or just possibly play some tricks to 100% force gcc not to
touch memory anywhere inside dcache_disable() or the functions it calls.
We're just getting lucky here right now since everything happens to be
inlined, but I don't think we're doing anything to 100% guarantee this.
What worries me here is that at least on Tegra, a "flush the entire
dcache" operation requires an SMC call to the secure monitor. That will
certainly access DRAM when the secure monitor runs, but perhaps this
doesn't matter since that's at a different exception level, and we know
the secure monitor accesses DRAM regions that are separate from U-Boot's
DRAM? I suspect life isn't that convenient. I'm wondering if this all
implies that, like patch 2 in this series, we need to get 100% away from
flush-by-set/way, even with SoC-specific hooks to make that work
reliably, and just flush everything by VA, which IIRC is architecturally
guaranteed to work without SoC-specific logic. That way, we can
encapsulate everything into an assembly function without worrying about
calling SMCs or SoC-specific hook functions without using DRAM. Of
course, how that assembly function knows which VAs to flush without
decoding the page tables or other data structure is another matter:-(
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