Good point, but the problem is that this isn't x86. People are assuming when they develop:
1. A15 always means LPAE (which it does unless you want to do multiplatform)
2. That you won't try to do A8/A9 and A15 simultaneously.
Sure, it's unfortunate, but that is why I don't think we will have much luck trying to maintain a multiplatform A8/A9/A15 vs. A8/A9 and LPAE in the longer term. For this specific errata we might be able to hack something (well sure, I can hack it to patch that function at runtime during boot so that it doesn't execute on non-A15 systems) but generally I feel it's a lost cause.
Jon.
--
Sent from my iPad
On Jun 28, 2013, at 1:41, Peter Robinson <pbrobinson@xxxxxxxxx> wrote:
> On Fri, Jun 28, 2013 at 5:09 AM, Jon Masters <jonathan@xxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> -------- Original Message --------
>> Subject: dummy_flush_tlb_a15_erratum in check_and_switch_context
>> Date: Thu, 27 Jun 2013 23:48:14 -0400
>> From: Jon Masters <jonathan@xxxxxxxxxxxxxx>
>> Organization: World Organi{s,z}ation of Broken Dreams
>> To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>
>> Hi Folks,
>>
>> Post mostly for Google's benefit. Fedora folks were reporting the
>> following backtrace on Cortex-A8 OMAP:
>>
>> [ 12.182873] Internal error: Oops - undefined instruction: 0 [#1] SMP ARM
>> [ 12.189971] Modules linked in: drm_kms_helper drm
>> [ 12.194965] CPU: 0 PID: 153 Comm: dracut-initqueu Not tainted
>> 3.10.0-0.rc7.git0.2.fc20.armv7hl #1
>> [ 12.204317] task: c9ee9b80 ti: c9f50000 task.ti: c9f50000
>> [ 12.210025] PC is at check_and_switch_context+0x3c0/0x44c
>> [ 12.215724] LR is at check_and_switch_context+0x364/0x44c
>> [ 12.221424] pc : [<c001dbd4>] lr : [<c001db78>] psr: 400f0093
>> [ 12.221424] sp : c9f51e40 ip : 00000000 fp : c9ebe860
>> [ 12.233532] r10: c08cb470 r9 : c08d97c8 r8 : c9ebe700
>> [ 12.239044] r7 : 00000000 r6 : 00000200 r5 : 00000000 r4 : 00000201
>> [ 12.245929] r3 : 00000000 r2 : 00000000 r1 : 00000000 r0 : 00000001
>> [ 12.252817] Flags: nZcv IRQs off FIQs on Mode SVC_32 ISA ARM
>> Segment user
>> [ 12.260436] Control: 10c5387d Table: 80004019 DAC: 00000015
>> [ 12.266497] Process dracut-initqueu (pid: 153, stack limit = 0xc9f50240)
>> [ 12.273568] Stack: (0xc9f51e40 to 0xc9f52000)
>> [ 12.278174] 1e40: c08cb478 00000000 00000200 00000000 200f0093
>> c08d853c c9f41e00 c9ebe380
>> [ 12.286808] 1e60: 00000000 c9ee9b80 c0c7db80 c9f50000 c9eeb700
>> c9ebe700 c9f51f24 c05994b8
>> [ 12.295440] 1e80: 00000004 c0250920 00000004 c0047954 d6266cd2
>> 00000002 00000000 00000000
>> [ 12.304074] 1ea0: 00000000 c0048854 c08cdb80 003b0000 d6266cd2
>> 00000002 00006ae1 c007a670
>> [ 12.312708] 1ec0: 00000139 00000000 0000b40e 0000b40e 00006a0d
>> c007a670 f5257d14 c0079274
>> [ 12.321342] 1ee0: c9f41e00 00000000 00000003 0000081f c08ded98
>> bea94f88 c9f51fb0 000cf704
>> [ 12.329976] 1f00: c9f51f84 c9f51f60 c9f50028 c9ee9b80 00000000
>> c9f51f78 fffffff6 c9f50000
>> [ 12.338598] 1f20: c9f50000 c0048854 c9ee9dcc c9eeb700 c9f51f38
>> c9ee9e14 00000000 00000000
>> [ 12.347220] 1f40: 00000004 00000000 00000000 bea951a8 c9f50000
>> 00000000 000d6d64 c004988c
>> [ 12.355841] 1f60: 00000003 00000004 00000000 00000000 bea951a8
>> 00000000 00000000 c9ee9b80
>> [ 12.364463] 1f80: c0047438 c9eeedd0 c9eeedd0 00000000 00000000
>> bea951a8 ffffffff 00000072
>> [ 12.373084] 1fa0: c000e344 c000e1a0 00000000 bea951a8 ffffffff
>> bea951a8 00000000 00000000
>> [ 12.381705] 1fc0: 00000000 bea951a8 ffffffff 00000072 000cf704
>> 000d6094 00000000 000d6d64
>> [ 12.390328] 1fe0: 000cf164 bea95158 00045180 b6e37ae0 600f0010
>> ffffffff 2d10a02c c8542a0a
>> [ 12.398987] [<c001dbd4>] (check_and_switch_context+0x3c0/0x44c) from
>> [<c05994b8>] (__schedule+0x4ac/0x750)
>> [ 12.409193] [<c05994b8>] (__schedule+0x4ac/0x750) from [<c0048854>]
>> (do_wait+0x1ec/0x244)
>> [ 12.417834] [<c0048854>] (do_wait+0x1ec/0x244) from [<c004988c>]
>> (SyS_wait4+0xa8/0xc8)
>> [ 12.426206] [<c004988c>] (SyS_wait4+0xa8/0xc8) from [<c000e1a0>]
>> (ret_fast_syscall+0x0/0x30)
>> [ 12.435116] Code: 1e082f13 f57ff04f f57ff06f e3a03000 (ee083f33)
>> [ 12.441552] ---[ end trace c0816de7f5b496a8 ]---
>>
>> I disassembled that faulting instruction manually just now, and it
>> appears to be:
>>
>> 1110 1110 000 0 1000 0011 1111 001 1 0011
>> opc1 CRn Rt coproc opc2 CRm
>>
>> MCR cp15, 0, r3, c8, c3, 1
>>
>> Which maps back to the call to dummt_flush_tlb_a15_erratum in
>> check_and_switch_context:
>>
>> #ifdef CONFIG_ARM_ERRATA_798181
>> static inline void dummy_flush_tlb_a15_erratum(void)
>> {
>> /*
>> * Dummy TLBIMVAIS. Using the unmapped address 0 and ASID 0.
>> */
>> asm("mcr p15, 0, %0, c8, c3, 1" : : "r" (0));
>> dsb();
>> }
>> #else
>> static inline void dummy_flush_tlb_a15_erratum(void)
>> {
>> }
>> #endif
>>
>> Now I think it's personally just easier to only turn on that errata on
>> LPAE/A15 kernels and just leave it at that (I've requested this get
>> moved to the lpae config and out of the base config so this is what
>> should happen shortly - clearly the intention), but some folks out there
>> want to do exciting things...I got asked if this could be runtime
>> patched (which I guess in theory is possible), but I'm not going there.
>>
>> Anyway, in addition, does this kind of thing need fixing with a more
>> specific Kconfig so that there's an explicit A15 dependency in there?
>> Rather just "depends on CPU_V7 && SMP"?
>
> I think it needs to be run time detectable, according to Arnd if you
> have a A15 kernel with < 4gb of RAM you don't want to run a LPAE
> kernel as it's got quite decent performance penalties. I suppose the
> other question to be asked is that as it's for A15 revisions
> r0p0..r3p2 is whether how much of that silicon is about and if it was
> never really widely available if we just disable it all together.
>
> Peter
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