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. -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
