[fedora-arm] Fwd: dummy_flush_tlb_a15_erratum in check_and_switch_context

[Peter Robinson]

On Fri, Jun 28, 2013 at 5:09 AM, Jon Masters <jonathan at jonmasters.org> 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 at jonmasters.org>
> Organization: World Organi{s,z}ation of Broken Dreams
> To: linux-arm-kernel at lists.infradead.org
>
> 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