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    SubjectRe: [PATCH 04/15] perf: Add ability to dump user regs
    On Wed, Mar 28, 2012 at 11:12:30AM -0400, Frank Ch. Eigler wrote:
    > Hi, Jiri -
    >
    > > [...]
    > > > [...] 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. [...]
    > >
    > > 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.
    >
    > I believe that's the right area. I'm not sure even the 32-bit variant
    > is complete enough, for example exempting MMX/SSE registers. These
    > may also contain spilled registers before long.
    >
    >
    > > 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.
    >
    > Yeah; the question is how much is missed besides those "most cases".
    >
    >
    > > > To recover these registers at run time, we found that the kernel
    > > > stack itself has to be partially unwound [... Without that, it ...]
    > > > may accidentally pass garbage data to perf userspace. Correcting
    > > > this could require a kernel-space libunwind.
    >
    > > AFAIK not going to happen any time soon ;)
    >
    > Understood. Then the code needs to ensure that it does not purport to
    > pass register values that it does not know. (Back when we were at
    > this stage in systemtap, we got some reasonable backtraces even
    > without kernel unwinding, ie. tolerating missing registers.)

    Right.

    I think in normal syscall case we save rdi, rsi, rdx, rax and rip.
    If we take the syscall slow path we save rbx, rbp, r12-15.

    Unfortunately we don't save rsp, which must be the most important
    for cfi unwinding.

    We probably need to check what is saved in irqs (set_irq_regs())
    and exceptions as well.


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