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    SubjectRe: [RFC PATCH 1/2] x86: Fix rbp saving in pt_regs on irq entry
    >>> On 06.01.11 at 18:12, Frederic Weisbecker <fweisbec@gmail.com> wrote:
    > On Thu, Jan 06, 2011 at 04:58:54PM +0000, Jan Beulich wrote:
    >> >>> On 06.01.11 at 17:54, Frederic Weisbecker <fweisbec@gmail.com> wrote:
    >> > On Thu, Jan 06, 2011 at 04:39:39PM +0000, Jan Beulich wrote:
    >> >> >>> On 06.01.11 at 17:22, Frederic Weisbecker <fweisbec@gmail.com> wrote:
    >> >> > On Thu, Jan 06, 2011 at 04:10:55PM +0000, Jan Beulich wrote:
    >> >> >> >>> On 06.01.11 at 16:45, Frederic Weisbecker <fweisbec@gmail.com> wrote:
    >> >> >> > Before we had:
    >> >> >> >
    >> >> >> >
    >> >> >> > leaveq
    >> >> >> >
    >> >> >> > CFI_RESTORE rbp
    >> >> >> > CFI_DEF_CFA_REGISTER rsp
    >> >> >> > CFI_ADJUST_CFA_OFFSET -8
    >> >> >> >
    >> >> >> > So CFI_RESTORE means rbp has now the value of the base frame of
    >> >> >> > the calling frame (the base frame pointer of the interrupted proc) ?
    >> >> >>
    >> >> >> No - all it means is that %rbp now has its original (caller or
    >> >> >> interrupted procedure) value again (i.e. an unwinder should not
    >> >> >> try to read it from the stack [or other previously recorded
    >> >> >> location] anymore).
    >> >> >>
    >> >> >> > And what follows means that rsp-8 points to the return address?
    >> >> >>
    >> >> >> No - .cfi_def_cfa_register says which register serves as the frame
    >> >> >> pointer, and .cfi_adjust_cfa_offset says to adjust the offset from
    >> >> >> the frame pointer to the top [or bottom] of frame. At any time
    >> >> >>
    >> >> >> CFA = cfa_register + cfa_offset
    >> >> >>
    >> >> >> with CFA being what all locations on the stack are expressed
    >> >> >> relative to.
    >> >> >
    >> >> > Ok.
    >> >> >
    >> >> > So here rsp points to pt_regs::r11
    >> >> >
    >> >> > I don't understand why locations relative to the stack must be
    >> >> > expressed here by taking rsp - 8 as a base.
    >> >>
    >> >> Nothing says rsp-8. The annotations merely say to set the base
    >> >> register to rsp and to *adjust* the offset by -8 (after all, that's
    >> >> what the leaveq instruction does).
    >> >
    >> > Ah! So CFA acts like a virtual frame base pointer right?
    >>
    >> Correct.
    >
    > Ah great. I was starting to prepare for the case you come to stab me :)
    >
    > So what do you think about that:
    >
    > leaveq
    >
    > CFI_RESTORE rbp
    > CFI_DEF_CFA_REGISTER rsp
    > CFI_ADJUST_CFA_OFFSET -8
    >
    > /* we did not save rbx, restore only from ARGOFFSET */
    > addq $8, %rsp
    > CFI_ADJUST_CFA_OFFSET -16
    >
    >
    > Does that look correct to you? We increased rsp to start recovering
    > the regs from the right place, but the frame pointer of the current
    > proc must stay what it was.

    As you hinted in your subsequent reply - it's -8 here (that's
    why the directive is named *adjust*; there are other
    directives allowing to *set* an offset).

    > Now I don't understand how this is all useful as this is not a normal
    > proc but an interruption. We can't get back the return address from
    > the CFA. Or am I missing something?

    Unwind annotations, when written correctly, allow unwinding
    through all kinds of execution flows, including interrupts or
    exceptions as well as including stack switches.

    Jan



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