Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2016-02-17 10:55:50
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On Wed, 2016-02-10 at 17:21 +0100, Torsten Duwe wrote:
The gcc switch -mprofile-kernel, available for ppc64 on gcc > 4.8.5, allows to call _mcount very early in the function, which low-level ASM code and code patching functions need to consider. Especially the link register and the parameter registers are still alive and not yet saved into a new stack frame.
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quoted hunk ↗ jump to hunk
diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c index ac64ffd..72a1a52 100644 --- a/arch/powerpc/kernel/module_64.c +++ b/arch/powerpc/kernel/module_64.c@@ -476,17 +474,44 @@ static unsigned long stub_for_addr(Elf64_Shdr *sechdrs, return (unsigned long)&stubs[i]; } +#ifdef CC_USING_MPROFILE_KERNEL +static int is_early_mcount_callsite(u32 *instruction) +{ + /* -mprofile-kernel sequence starting with + * mflr r0 and maybe std r0, LRSAVE(r1). + */ + if ((instruction[-3] == PPC_INST_MFLR && + instruction[-2] == PPC_INST_STD_LR) || + instruction[-2] == PPC_INST_MFLR) { + /* Nothing to be done here, it's an _mcount + * call location and r2 will have to be + * restored in the _mcount function. + */ + return 1; + } + return 0; +}
So this logic to deal with the 2 vs 3 instruction version of the mcount sequence is problematic. On a kernel built with the 2 instruction version this will fault when the function we're looking at is located at the beginning of a page. Because instruction[-3] goes off the front of the mapping. We can probably fix that. But it's still a bit dicey. I'm wondering if we want to just say we only support the 2 instruction version. Currently that means GCC 6 only, or a distro compiler with the backport of e95d0248dace. But we could also ask GCC to backport it to 4.9 and 5. Thoughts? cheers