Messages in this thread | | | From | Andy Lutomirski <> | Date | Sat, 10 Jan 2015 14:04:46 -0800 | Subject | Re: [PATCH 3/4] x86: open-code register save/restore in trace_hardirqs thunks |
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On Sat, Jan 10, 2015 at 2:00 PM, Borislav Petkov <bp@alien8.de> wrote: > On Sat, Jan 10, 2015 at 09:14:03PM +0100, Denys Vlasenko wrote: >> From 2f636e0a92db898f2bdb592027aa302fcb32a326 Mon Sep 17 00:00:00 2001 >> From: Denys Vlasenko <dvlasenk@redhat.com> >> To: linux-kernel@vger.kernel.org >> Subject: [PATCH 3/4] x86: open-code register save/restore in trace_hardirqs thunks >> >> This is a preparatory patch for change in "struct pt_regs" >> handling in entry_64.S. >> >> trace_hardirqs thunks were (ab)using a part of pt_regs >> handling code, namely SAVE_ARGS/RESTORE_ARGS macros, >> to save/restore registers across C function calls. >> >> Since SAVE_ARGS is going to be changed, open-code >> register saving/restoring here. Take a page from thunk_32.S >> and use push/pop insns instead of movq, they are far shorter: >> 1 or 2 bytes versus 5, and no need for insns to adjust %rsp: >> >> text data bss dec hex filename >> 333 40 0 373 175 thunk_64_movq.o >> 104 40 0 144 90 thunk_64_push_pop.o >> >> Incidentally, this removes a bit of dead code: >> one SAVE_ARGS was used just to emit a CFI annotation, >> but it also generated unreachable assembly insns. >> >> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com> >> CC: Linus Torvalds <torvalds@linux-foundation.org> >> CC: Oleg Nesterov <oleg@redhat.com> >> CC: "H. Peter Anvin" <hpa@zytor.com> >> CC: Borislav Petkov <bp@alien8.de> >> CC: Andy Lutomirski <luto@amacapital.net> >> CC: Frederic Weisbecker <fweisbec@gmail.com> >> CC: X86 ML <x86@kernel.org> >> CC: Alexei Starovoitov <ast@plumgrid.com> >> CC: Will Drewry <wad@chromium.org> >> CC: Kees Cook <keescook@chromium.org> >> CC: linux-kernel@vger.kernel.org >> --- >> arch/x86/lib/thunk_64.S | 46 ++++++++++++++++++++++++++++++++++++++++++---- >> 1 file changed, 42 insertions(+), 4 deletions(-) >> >> diff --git a/arch/x86/lib/thunk_64.S b/arch/x86/lib/thunk_64.S >> index b30b5eb..8ec443a 100644 >> --- a/arch/x86/lib/thunk_64.S >> +++ b/arch/x86/lib/thunk_64.S >> @@ -17,9 +17,27 @@ >> CFI_STARTPROC >> >> /* this one pushes 9 elems, the next one would be %rIP */ >> - SAVE_ARGS >> + pushq_cfi %rdi >> + CFI_REL_OFFSET rdi, 0 > > Btw, why the second CFI annotation? > > pushq_cfi does already CFI_ADJUST_CFA_OFFSET 8. Can't we use one and > hide it in the macro?
By my imperfect understanding of CFI:
CFI_ADJUST_CFA_OFFSET means that the offset between rsp and the "canonical frame address" is increased by 8 (because we just subtracted 8 from rsp) and CFI_REL_OFFSET reg, 0 means that the unwinder can find reg at offset 0 + (cfa offset here) from the CFA.
IOW, one is to keep the stack frame tracking consistent and the other is to tell the unwinder about the register we just saved.
--Andy
> > Btw, patch boots fine in the guest. > > -- > Regards/Gruss, > Boris. > > Sent from a fat crate under my desk. Formatting is fine. > --
-- Andy Lutomirski AMA Capital Management, LLC
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