Messages in this thread | | | Date | Fri, 07 Jan 2011 07:45:53 +0000 | From | "Jan Beulich" <> | Subject | Re: [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
| |