Re: framebuffer corruption due to overlapping stp instructions on arm64

[Ard Biesheuvel]

On 6 August 2018 at 21:54, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
>
> On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
>
>> 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
>
> Unfortunatelly, it doesn't work. I verified that the bit is set after
> booting Linux, but the memcpy corruption was still present.
>
> I also tried the other chicken bits, it slowed down the system noticeably,
> but had no effect on the memcpy corruption.
>

OK, it was worth a shot

Let's wait and see if Marcin has any results.