Re: [PATCH 1/2] Fix perf LBR filtering

From: Peter Zijlstra
Date: Thu Apr 25 2013 - 12:27:05 EST

On Wed, Apr 24, 2013 at 04:04:53PM -0700, Andi Kleen wrote:
> Possible options:
> I) Disable FAR calls for ANY_CALL/RETURNS.
> This just means syscalls are not logged
> as calls. This also lowers the overhead of call logging.
> This changes semantics slightly.
> This is reasonable on Sandy Bridge and later, but would
> cause additional problems on Nehalem and Westmere with
> their additional filters.
> II) Simple disable any filtering for kernel space.
> This means interrupts in kernel space are reported as calls
> and on Nehalem/Westmere some indirect jumps are reported
> as calls too
> III) Enumerate all the kernel entry points and check.
> Any bad call must have a kernel entry point as to.
> This seemed to fragile to maintain.
> IV) Enumerate all kernel code and check for these ranges.
> Quite complicated, especially with the new kernel code JITs.
> Would also allow to probe for kernel code (defeating randomized kernel)

So why not do the same as we do for userspace? Copy MAX_INSN_SIZE bytes
and trap -EFAULT.

With Steven's recent NMI nesting stuff we should be able to take the fault and
do the fixup_exception() thing. Or alternatively we could software walk the
kernel pagetables.

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