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    SubjectRe: [PATCH 04/15] perf: Add ability to dump user regs
    On Wed, Mar 28, 2012 at 10:01:15AM -0400, Frank Ch. Eigler wrote:
    > Hi, Jiri -
    > On Wed, Mar 28, 2012 at 02:35:47PM +0200, Jiri Olsa wrote:
    > > [...]
    > > The register value here are those of the user space context as
    > > it was before the user entered the kernel for whatever reason
    > > (syscall, irq, exception, or a PMI happening in userspace).
    > > [...]
    > As I understand the situation, there is a complication here that you
    > haven't accounted for. Upon a normal syscall entry to the kernel, not
    > all user registers are saved explicitly for such easy retrieval. The
    > others may be spilled to the stack by gcc during the various sys_*
    > functions or elsewhere. It turns out that some of these saved
    > registers are sometimes necessary to accomplish a user-space unwind.

    Are you reffering to x86_64 where only portion of registers
    is stored by SAVE_ARGS macro? Seems like 32 bits stores the
    whole pt_regs.

    Generally you could need all the registers to start the unwind,
    but I was assuming that for most cases the stack pointer and
    instruction pointer should be enough.. but I might be wrong here.

    > To recover these registers at run time, we found that the kernel stack
    > itself has to be partially unwound - and not via frame pointers, but
    > the full dwarf unwind/cfi machinery. This RFC code does not appear
    > aware of the difference between the explicitly saved and the
    > incidentally-spilled registers, and thus may accidentally pass garbage
    > data to perf userspace. Correcting this could require a kernel-space
    > libunwind.

    AFAIK not going to happen any time soon ;)


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