Re: framebuffer corruption due to overlapping stp instructions on arm64

[Ard Biesheuvel]

On 6 August 2018 at 19:09, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
>
> On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
>
>> On 6 August 2018 at 14:42, Robin Murphy <robin.murphy@xxxxxxx> wrote:
>> > On 06/08/18 11:25, Mikulas Patocka wrote:
>> > [...]
>> >>>
>> >>> None of this explains why some transactions fail to make it across
>> >>> entirely. The overlapping writes in question write the same data to
>> >>> the memory locations that are covered by both, and so the ordering in
>> >>> which the transactions are received should not affect the outcome.
>> >>
>> >>
>> >> You're right that the corruption couldn't be explained just by reordering
>> >> writes. My hypothesis is that the PCIe controller tries to disambiguate
>> >> the overlapping writes, but the disambiguation logic was not tested and it
>> >> is buggy. If there's a barrier between the overlapping writes, the PCIe
>> >> controller won't see any overlapping writes, so it won't trigger the
>> >> faulty disambiguation logic and it works.
>> >>
>> >> Could the ARM engineers look if there's some chicken bit in Cortex-A72
>> >> that could insert barriers between non-cached writes automatically?
>> >
>> >
>> > I don't think there is, and even if there was I imagine it would have a
>> > pretty hideous effect on non-coherent DMA buffers and the various other
>> > places in which we have Normal-NC mappings of actual system RAM.
>> >
>>
>> Looking at the A72 manual, there is one chicken bit that looks like it
>> may be related:
>>
>> CPUACTLR_EL1 bit #50:
>>
>> 0 Enables store streaming on NC/GRE memory type. This is the reset value.
>> 1 Disables store streaming on NC/GRE memory type.
>>
>> so putting something like
>>
>> mrs x0, S3_1_C15_C2_0
>> orr x0, x0, #(1 << 50)
>> msr S3_1_C15_C2_0, x0
>>
>> in __cpu_setup() would be worth a try.
>
> It won't boot.
>
> But if i write the same value that was read, it also won't boot.
>
> I created a simple kernel module that reads this register and it has bit
> 32 set, all other bits clear. But when I write the same value into it, the
> core that does the write is stuck in infinite loop.
>
> So, it seems that we are writing this register from a wrong place.
>

Ah, my bad. I didn't look closely enough at the description:

"""
The accessibility to the CPUACTLR_EL1 by Exception level is:

EL0              -
EL1(NS)          RW (a)
EL1(S)           RW (a)
EL2              RW (b)
EL3(SCR.NS = 1)  RW
EL3(SCR.NS = 0)  RW

(a) Write access if ACTLR_EL3.CPUACTLR is 1 and ACTLR_EL2.CPUACTLR is
1, or ACTLR_EL3.CPUACTLR is 1 and SCR.NS is 0.
"""

so you'll have to do this from ARM Trusted Firmware. If you're
comfortable rebuilding that:

diff --git a/include/lib/cpus/aarch64/cortex_a72.h
b/include/lib/cpus/aarch64/cortex_a72.h
index bfd64918625b..a7b8cf4be0c6 100644
--- a/include/lib/cpus/aarch64/cortex_a72.h
+++ b/include/lib/cpus/aarch64/cortex_a72.h
@@ -31,6 +31,7 @@
 #define CORTEX_A72_ACTLR_EL1                   S3_1_C15_C2_0

 #define CORTEX_A72_ACTLR_DISABLE_L1_DCACHE_HW_PFTCH    (1 << 56)
+#define CORTEX_A72_ACTLR_DIS_NC_GRE_STORE_STREAMING    (1 << 50)
 #define CORTEX_A72_ACTLR_NO_ALLOC_WBWA         (1 << 49)
 #define CORTEX_A72_ACTLR_DCC_AS_DCCI           (1 << 44)
 #define CORTEX_A72_ACTLR_EL1_DIS_INSTR_PREFETCH                (1 << 32)
diff --git a/lib/cpus/aarch64/cortex_a72.S b/lib/cpus/aarch64/cortex_a72.S
index 55e508678284..5914d6ee3ba6 100644
--- a/lib/cpus/aarch64/cortex_a72.S
+++ b/lib/cpus/aarch64/cortex_a72.S
@@ -133,6 +133,15 @@ func cortex_a72_reset_func
        orr     x0, x0, #CORTEX_A72_ECTLR_SMP_BIT
        msr     CORTEX_A72_ECTLR_EL1, x0
        isb
+
+       /* ---------------------------------------------
+        * Disables store streaming on NC/GRE memory type.
+        * ---------------------------------------------
+        */
+       mrs     x0, CORTEX_A72_ACTLR_EL1
+       orr     x0, x0, #CORTEX_A72_ACTLR_DIS_NC_GRE_STORE_STREAMING
+       msr     CORTEX_A72_ACTLR_EL1, x0
+       isb
        ret x19
 endfunc cortex_a72_reset_func