On 25/4/19 11:57, Lucas Stach wrote:
> [T]he sequence that could go wrong in Barebox is as follows:
> 1. CPU core starts invalidating at L1 cache level
> 2. HW prefetcher decides that a specific address should be brought into
> the L1 cache.
> 3. HW prefetcher finds a valid block for the requested address in L2
> cache and moves cached data from L2 to L1.
> 4. Only now CPU core invalidates L2 cache.
>
> In the above sequence we now have invalid data in the L1 cache line.
> The correct sequence will avoid this issue:
>
> 1. CPU core starts invalidating at L2 cache level
> 2. HW prefetcher decides that a specific address should be brought into
> the L1 cache.
> 3. HW prefetcher sees invalid tags for the requested address in L2
> cache and brings in the data from external memory.
> 4. CPU core invalidates L1 cache, discarding the prefetched data.
>
The ARM Cortex-A Series Programmer's Guide addresses this issue
in the SMP-context[1]:
> If another core were to access the affected address between those
> two actions, a coherency problem can occur. Such problems can be avoided
> by following two simple rules.
>
> * When cleaning, always clean the innermost (L1) cache first and then
> clean the outer cache(s).
> * When invalidating, always invalidate the outermost cache first and
> the L1 cache last.
The current code correctly iterates from inner to outer cache levels
when flushing/cleaning (r8 == 0), invalidation (r8 == 1) occurs in the
same direction though. Adjust the invalidation iteration order to start
from the outermost cache instead.
Equivalent C-Code:
enum cache_op { CACHE_FLUSH = 0, CACHE_INVAL = 1 };
register enum cache_op operation asm("r8");
register int i asm("r12");
register int limit asm("r3") = max_cache_level << 1; // e.g. 4 with L2 max
+if (operation == CACHE_FLUSH) {
i = 0;
+} else {
+ i = limit - 2;
+}
bool loop_again;
do {
/* [snip] */
+ if (operation == CACHE_FLUSH) {
i += 2;
loop_again = limit > i;
+ } else {
+ loop_again = i > 0;
+ i -= 2;
+ }
} while (loop_again);
[1]: 18.6 "TLB and cache maintenance broadcast", Version 4.0
Suggested-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>
---
arch/arm/cpu/cache-armv7.S | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/arch/arm/cpu/cache-armv7.S b/arch/arm/cpu/cache-armv7.S
index 9487dd03f6cc..21e189cda1d7 100644
--- a/arch/arm/cpu/cache-armv7.S
+++ b/arch/arm/cpu/cache-armv7.S
@@ -83,7 +83,10 @@ hierarchical:
ands r3, r0, #0x7000000 @ extract loc from clidr
mov r3, r3, lsr #23 @ left align loc bit field
beq finished @ if loc is 0, then no need to clean
- mov r12, #0 @ start clean at cache level 0
+ cmp r8, #0
+THUMB( ite eq )
+ moveq r12, #0
+ subne r12, r3, #2 @ start invalidate at outmost cache level
loop1:
add r2, r12, r12, lsr #1 @ work out 3x current cache level
mov r1, r0, lsr r2 @ extract cache type bits from clidr
@@ -118,9 +121,16 @@ THUMB( ite eq )
subs r7, r7, #1 @ decrement the index
bge loop2
skip:
+ cmp r8, #0
+ bne inval_check
add r12, r12, #2 @ increment cache number
cmp r3, r12
- dsb @ work-around Cortex-A7 erratum 814220
+ b loop_end_check
+inval_check:
+ cmp r12, #0
+ sub r12, r12, #2 @ decrement cache number
+loop_end_check:
+ dsb
bgt loop1
finished:
ldmfd sp!, {r4-r11}
--
2.20.1
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