Re: [PATCH] arm: add an option for erratum 657417

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On 12 August 2016 at 15:49, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> On 12 August 2016 at 15:15, Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
>> On Fri, 12 Aug 2016 13:33:14 +0100
>> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote:
>>
>>> On Fri, Aug 12, 2016 at 06:19:17PM +1000, Nicholas Piggin wrote:
>>> > This patch adds an option which defaults to "y" in cases where we
>>> > could possibly be running Cortex A8 and using Thumb2 instructions.
>>> > In reality the workaround might not be required at all for the kernel
>>> > if virtual instruction memory is linear in physical memory.
>>>
>>> Hmm.
>>>
>>> The main kernel image is guaranteed to be contiguous in physical memory
>>> for all sorts of reasons, so this really isn't a concern for the kernel
>>> itself.
>>
>> That's what it *seems* like. I wanted to be conservative because I don't
>> know the architecture nor have actually looked at the errata docs. You
>> can probably make stronger guarantees to avoid it. Perhaps enabling just
>> for modules would be workable.
>>
>>
>>> Modules, however, are a different matter, as they are mapped in using
>>> individual pages, and are most likely to be non-contiguous in physical
>>> memory.  The kernel's module linker knows nothing about this errata,
>>> so it'll generally just fix up the relocations in the most basic of
>>> ways.
>>>
>>> So, I think we should always use this --no-fix-cortex-a8 option where
>>> the linker supports it irrespective of whether we're running on a core
>>> needing this workaround, but we probably need to fix the kernel module
>>> linker to know about this.
>>
>> It looks like it would be a bit of work to go that route. The linker of
>> course would not give you relocations or stubs for the branches you
>> need them.
>>
>
> We could enable CONFIG_ARM_MODULE_PLTS in this case, and force a
> branch via a PLT entry if an affected instruction is encountered.
> However, this only covers branch instructions that are covered by
> relocations, so we'd still need to scan the module .text to look for
> affected instructions whose targets has been resolved at compile time.
>
> Running this

$ objdump -dr vmlinux |grep -A1 -E \\sb\.w |less

I get numerous instances of b.w that are not covered by any
relocations, so i assume that will be the case for modules as well.
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