Maynard,
Thanks for the patch. I don't know the utrace API but I have a few comments
about your patch:
- it looks like ptrace remains available to users even though utrace
is in the kernel.
I am assuming ptrace is then built on top of utrace.
- the current perfmon API requires that the user issue
ptrace(ATTACH) before allowing
access to the perfmon context from another thread. This is because
certain perfmon
actions require modifying the PMU and/or machine state of the other thread.
- The monitored thread must be off any CPU completely before we can
proceed with
a perfmon access. Although there is locking around the perfmon
context and lock
is grabbed when context switching, there are certain
architectures, e.g., Itanium,
where perfmon call need to modifying state beyond the pure PMU
registers, e.g.,
a processor status register which is not protected by any perfmon lock.
- The thread must remain off any CPU during the entire duration of
the perfmon access.
It should not be possible to wake up the while in the middle of a
perfmon syscall. That
is one property of ptrace we heavily leverage. Only the ptracing
parent can wake the
thread.
- To maintain compatibility with existing perfmon applications using
ptrace, you have to
fail is you notice the thread is not in a quiescent state or is
not going to. It would be
nice if the kernel could stop the thread on behalf of the caller.
This is not very easy to
do with ptrace, maybe with utrace it is. I am willing to revisit
this as I think it would be
nicer to user. Ptrace has other undesirable side effects, such as
redirecting signals.
- Suppose I would like to add your patch to the existing tree, what
CONFIG_XXX should
I use to make this code compile only of utrace is enabled?
Thanks.
On Fri, Mar 28, 2008 at 10:04 PM, Maynard Johnson <[EMAIL PROTECTED]> wrote:
> Hi, Stephane,
> As you may recall, about a month ago, I posted a question to the utrace
> mailing list, asking about a compile failure I was seeing in
> perfmon/perfmon_syscalls.c because it's trying to use the function
> ptrace_check_attach (removed by the utrace patch). I've attached a
> patch that we've been using that fixes the compile failure and seems to
> functionally work. Feel free to use it if you wish. Please let me know
> if it seems correct to you.
>
> Thanks.
>
> Maynard Johnson
> IBM POWER Toolchain Team
>
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