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SubjectRe: [PATCH 11/12] hw-breakpoints: ftrace plugin for kernel symbol tracing using HW Breakpoint interfaces
Frederic Weisbecker wrote:
> From: K.Prasad <prasad@linux.vnet.ibm.com>
>
> This patch adds an ftrace plugin to detect and profile memory access over kernel
> variables. It uses HW Breakpoint interfaces to 'watch memory addresses.
>
> Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> ---
> kernel/trace/Kconfig | 21 ++
> kernel/trace/Makefile | 1 +
> kernel/trace/trace.h | 23 ++
> kernel/trace/trace_ksym.c | 525 +++++++++++++++++++++++++++++++++++++++++
> kernel/trace/trace_selftest.c | 53 ++++
> 5 files changed, 623 insertions(+), 0 deletions(-)
> create mode 100644 kernel/trace/trace_ksym.c
[...]
> + entry->ksym_hbp->info.name = ksymname;
> + entry->ksym_hbp->info.type = op;
> + entry->ksym_addr = entry->ksym_hbp->info.address = addr;
> +#ifdef CONFIG_X86
> + entry->ksym_hbp->info.len = HW_BREAKPOINT_LEN_4;
> +#endif

What if the symbol referred to an object of size other than 4? This
would clearly be incorrect in that case.


> + entry->ksym_hbp->triggered = (void *)ksym_hbp_handler;
> +
> + ret = register_kernel_hw_breakpoint(entry->ksym_hbp);

I hate to sound like a broken record, but could some one explain to me
again why it is a good idea to design a new API that requires processor
specific #ifdefs to be sprinkled all around generic kernel code?

Back in:
http://lkml.org/lkml/2008/12/4/329
and
http://lkml.org/lkml/2009/5/21/189

I raised doubts about this hw-breakpoint thing being generic and the
responses made think that the processor specific portions would be
isolated in the processor specific parts of the kernel. I now see that
I was wrong.

When we add sparc, MIPS, ppc... Support it would be nice to not have to
add all our own #ifdefs to this, but instead have a generic interface
that will not need changes.

David Daney


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