Re: [PATCH 3/3] arm64: Add software workaround for Falkor erratum 1041

From: Shanker Donthineni
Date: Sat Nov 04 2017 - 17:44:00 EST

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In reply to: Robin Murphy: "Re: [PATCH 3/3] arm64: Add software workaround for Falkor erratum 1041"
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Hi Robin, Thanks for your review comments.

On 11/03/2017 10:11 AM, Robin Murphy wrote:
> On 03/11/17 03:27, Shanker Donthineni wrote:
>> The ARM architecture defines the memory locations that are permitted
>> to be accessed as the result of a speculative instruction fetch from
>> an exception level for which all stages of translation are disabled.
>> Specifically, the core is permitted to speculatively fetch from the
>> 4KB region containing the current program counter and next 4KB.
>>
>> When translation is changed from enabled to disabled for the running
>> exception level (SCTLR_ELn[M] changed from a value of 1 to 0), the
>> Falkor core may errantly speculatively access memory locations outside
>> of the 4KB region permitted by the architecture. The errant memory
>> access may lead to one of the following unexpected behaviors.
>>
>> 1) A System Error Interrupt (SEI) being raised by the Falkor core due
>> to the errant memory access attempting to access a region of memory
>> that is protected by a slave-side memory protection unit.
>> 2) Unpredictable device behavior due to a speculative read from device
>> memory. This behavior may only occur if the instruction cache is
>> disabled prior to or coincident with translation being changed from
>> enabled to disabled.
>>
>> To avoid the errant behavior, software must execute an ISB immediately
>> prior to executing the MSR that will change SCTLR_ELn[M] from 1 to 0.
>>
>> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
>> ---
>> Documentation/arm64/silicon-errata.txt | 1 +
>> arch/arm64/Kconfig | 10 ++++++++++
>> arch/arm64/include/asm/assembler.h | 17 +++++++++++++++++
>> arch/arm64/include/asm/cpucaps.h | 3 ++-
>> arch/arm64/kernel/cpu_errata.c | 16 ++++++++++++++++
>> arch/arm64/kernel/efi-entry.S | 4 ++--
>> arch/arm64/kernel/head.S | 4 ++--
>> 7 files changed, 50 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/arm64/silicon-errata.txt b/Documentation/arm64/silicon-errata.txt
>> index 66e8ce1..704770c0 100644
>> --- a/Documentation/arm64/silicon-errata.txt
>> +++ b/Documentation/arm64/silicon-errata.txt
>> @@ -74,3 +74,4 @@ stable kernels.
>> | Qualcomm Tech. | Falkor v1 | E1003 | QCOM_FALKOR_ERRATUM_1003 |
>> | Qualcomm Tech. | Falkor v1 | E1009 | QCOM_FALKOR_ERRATUM_1009 |
>> | Qualcomm Tech. | QDF2400 ITS | E0065 | QCOM_QDF2400_ERRATUM_0065 |
>> +| Qualcomm Tech. | Falkor v{1,2} | E1041 | QCOM_FALKOR_ERRATUM_1041 |
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 0df64a6..7e933fb 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -539,6 +539,16 @@ config QCOM_QDF2400_ERRATUM_0065
>>
>> If unsure, say Y.
>>
>> +config QCOM_FALKOR_ERRATUM_1041
>> + bool "Falkor E1041: Speculative instruction fetches might cause errant memory access"
>> + default y
>> + help
>> + Falkor CPU may speculatively fetch instructions from an improper
>> + memory location when MMU translation is changed from SCTLR_ELn[M]=1
>> + to SCTLR_ELn[M]=0. Prefix an ISB instruction to fix the problem.
>> +
>> + If unsure, say Y.
>> +
>> endmenu
>>
>>
>> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
>> index b6dfb4f..4c91efb 100644
>> --- a/arch/arm64/include/asm/assembler.h
>> +++ b/arch/arm64/include/asm/assembler.h
>> @@ -30,6 +30,7 @@
>> #include <asm/pgtable-hwdef.h>
>> #include <asm/ptrace.h>
>> #include <asm/thread_info.h>
>> +#include <asm/alternative.h>
>>
>> /*
>> * Enable and disable interrupts.
>> @@ -514,6 +515,22 @@
>> * reg: the value to be written.
>> */
>> .macro write_sctlr, eln, reg
>> +#ifdef CONFIG_QCOM_FALKOR_ERRATUM_1041
>> +alternative_if ARM64_WORKAROUND_QCOM_FALKOR_E1041
>> + tbnz \reg, #0, 8000f // enable MMU?
>
> Do we really need the branch here? It's not like enabling the MMU is
> something we do on the syscall fastpath, and I can't imagine an extra
> ISB hurts much (and is probably comparable to a mispredicted branch

I don't have any strong opinion on whether to use an ISB conditionally
or unconditionally. Yes, the current kernel code is not touching
SCTLR_ELn register on the system call fast path. I would like to keep
it as a conditional ISB in case if the future kernel accesses the
SCTLR_ELn on the fast path. An extra ISB should not hurt a lot but I
believe it has more overhead than the TBZ+branch mis-prediction on Falkor
CPU. This patch has been tested on the real hardware to fix the problem.

I'm open to change to an unconditional ISB if it's the better fix.

> anyway). In fact, is there any noticeable hit on other
> microarchitectures if we save the alternative bother and just do it
> unconditionally always?
>

I can't comment on the performance impacts of other CPUs since I don't
have access to their development platforms. I'll prefer alternatives
just to avoid the unnecessary overhead on future Qualcomm Datacenter
server CPUs and regression on other CPUs because of inserting an ISB
prior to SCTLR_ELn register update. Let's see what rest of the ARM
maintainers think about always using an ISB instead of alternatives.

> Robin.
>
>> + isb
>> +8000:
>> +alternative_else_nop_endif
>> +#endif
>> + msr sctlr_\eln, \reg
>> + .endm
>> +
>> + .macro early_write_sctlr, eln, reg
>> +#ifdef CONFIG_QCOM_FALKOR_ERRATUM_1041
>> + tbnz \reg, #0, 8000f // enable MMU?
>> + isb
>> +8000:
>> +#endif
>> msr sctlr_\eln, \reg
>> .endm
>>
>> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
>> index 8da6216..7f7a59d 100644
>> --- a/arch/arm64/include/asm/cpucaps.h
>> +++ b/arch/arm64/include/asm/cpucaps.h
>> @@ -40,7 +40,8 @@
>> #define ARM64_WORKAROUND_858921 19
>> #define ARM64_WORKAROUND_CAVIUM_30115 20
>> #define ARM64_HAS_DCPOP 21
>> +#define ARM64_WORKAROUND_QCOM_FALKOR_E1041 22
>>
>> -#define ARM64_NCAPS 22
>> +#define ARM64_NCAPS 23
>>
>> #endif /* __ASM_CPUCAPS_H */
>> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
>> index 0e27f86..27f9a45 100644
>> --- a/arch/arm64/kernel/cpu_errata.c
>> +++ b/arch/arm64/kernel/cpu_errata.c
>> @@ -179,6 +179,22 @@ static int cpu_enable_trap_ctr_access(void *__unused)
>> MIDR_CPU_VAR_REV(0, 0)),
>> },
>> #endif
>> +#ifdef CONFIG_QCOM_FALKOR_ERRATUM_1041
>> + {
>> + .desc = "Qualcomm Technologies Falkor erratum 1041",
>> + .capability = ARM64_WORKAROUND_QCOM_FALKOR_E1041,
>> + MIDR_RANGE(MIDR_QCOM_FALKOR_V1,
>> + MIDR_CPU_VAR_REV(0, 0),
>> + MIDR_CPU_VAR_REV(0, 0)),
>> + },
>> + {
>> + .desc = "Qualcomm Technologies Falkor erratum 1041",
>> + .capability = ARM64_WORKAROUND_QCOM_FALKOR_E1041,
>> + MIDR_RANGE(MIDR_QCOM_FALKOR,
>> + MIDR_CPU_VAR_REV(0, 1),
>> + MIDR_CPU_VAR_REV(0, 2)),
>> + },
>> +#endif
>> #ifdef CONFIG_ARM64_ERRATUM_858921
>> {
>> /* Cortex-A73 all versions */
>> diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
>> index acae627..c31be1b 100644
>> --- a/arch/arm64/kernel/efi-entry.S
>> +++ b/arch/arm64/kernel/efi-entry.S
>> @@ -96,14 +96,14 @@ ENTRY(entry)
>> read_sctlr el2, x0
>> bic x0, x0, #1 << 0 // clear SCTLR.M
>> bic x0, x0, #1 << 2 // clear SCTLR.C
>> - write_sctlr el2, x0
>> + early_write_sctlr el2, x0
>> isb
>> b 2f
>> 1:
>> read_sctlr el1, x0
>> bic x0, x0, #1 << 0 // clear SCTLR.M
>> bic x0, x0, #1 << 2 // clear SCTLR.C
>> - write_sctlr el1, x0
>> + early_write_sctlr el1, x0
>> isb
>> 2:
>> /* Jump to kernel entry point */
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index b8d5b73..9512ce7 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -511,7 +511,7 @@ install_el2_stub:
>> mov x0, #0x0800 // Set/clear RES{1,0} bits
>> CPU_BE( movk x0, #0x33d0, lsl #16 ) // Set EE and E0E on BE systems
>> CPU_LE( movk x0, #0x30d0, lsl #16 ) // Clear EE and E0E on LE systems
>> - write_sctlr el1, x0
>> + early_write_sctlr el1, x0
>>
>> /* Coprocessor traps. */
>> mov x0, #0x33ff
>> @@ -732,7 +732,7 @@ __primary_switch:
>> * to take into account by discarding the current kernel mapping and
>> * creating a new one.
>> */
>> - write_sctlr el1, x20 // disable the MMU
>> + early_write_sctlr el1, x20 // disable the MMU
>> isb
>> bl __create_page_tables // recreate kernel mapping
>>
>>
>
>
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>

--
Shanker Donthineni
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

Next message: Himanshu Jha: "[PATCH] firmware: raspberrypi: Eliminate use of time_to_tm"
Previous message: kbuild test robot: "Re: [PATCH 1/2] drivers: hwmon: Add W83773G driver"
In reply to: Robin Murphy: "Re: [PATCH 3/3] arm64: Add software workaround for Falkor erratum 1041"
Next in thread: Shanker Donthineni: "[PATCH 1/3] arm64: Define cputype macros for Falkor CPU"
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