On Thu, Feb 15, 2018 at 12:48 AM, Brian Gerst <brge...@gmail.com> wrote: > On Wed, Feb 14, 2018 at 7:17 PM, Andy Lutomirski <l...@kernel.org> wrote: >> On Wed, Feb 14, 2018 at 6:21 PM, Dominik Brodowski >> <li...@dominikbrodowski.net> wrote: >>> Moving the switch to IRQ stack from the interrupt macro to the helper >>> function requires some trickery: All ENTER_IRQ_STACK really cares about >>> is where the "original" stack -- meaning the GP registers etc. -- is >>> stored. Therefore, we need to offset the stored RSP value by 8 whenever >>> ENTER_IRQ_STACK is called from within a function. In such cases, and >>> after switching to the IRQ stack, we need to push the "original" return >>> address (i.e. the return address from the call to the interrupt entry >>> function) to the IRQ stack. >>> >>> This trickery allows us to carve another 1k from the text size: >>> >>> text data bss dec hex filename >>> 17905 0 0 17905 45f1 entry_64.o-orig >>> 16897 0 0 16897 4201 entry_64.o >>> >>> Signed-off-by: Dominik Brodowski <li...@dominikbrodowski.net> >>> --- >>> arch/x86/entry/entry_64.S | 53 >>> +++++++++++++++++++++++++++++++---------------- >>> 1 file changed, 35 insertions(+), 18 deletions(-) >>> >>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S >>> index de8a0da0d347..3046b12a1acb 100644 >>> --- a/arch/x86/entry/entry_64.S >>> +++ b/arch/x86/entry/entry_64.S >>> @@ -449,10 +449,18 @@ END(irq_entries_start) >>> * >>> * The invariant is that, if irq_count != -1, then the IRQ stack is in use. >>> */ >>> -.macro ENTER_IRQ_STACK regs=1 old_rsp >>> +.macro ENTER_IRQ_STACK regs=1 old_rsp save_ret=0 >>> DEBUG_ENTRY_ASSERT_IRQS_OFF >>> movq %rsp, \old_rsp >>> >>> + .if \save_ret >>> + /* >>> + * If save_ret is set, the original stack contains one additional >>> + * entry -- the return address. >>> + */ >>> + addq $8, \old_rsp >>> + .endif >>> + >> >> This is a bit alarming in that you now have live data below RSP. For >> x86_32, this would be a big no-no due to NMI. For x86_64, it might >> still be bad if there are code paths where NMI is switched to non-IST >> temporarily, which was the case at some point and might still be the >> case. (I think it is.) Remember that the x86_64 *kernel* ABI has no >> red zone. >> >> It also means that, if you manage to hit vmalloc_fault() in here when >> you touch the IRQ stack, you're dead. IOW you hit: >> >> movq \old_rsp, PER_CPU_VAR(irq_stack_union + IRQ_STACK_SIZE - 8) >> >> which gets #PF and eats your return pointer. Debugging this will be >> quite nasty because you'll only hit it on really huge systems after a >> thread gets migrated, and even then only if you get unlucky on your >> stack alignment. >> >> So can you find another way to do this? > > It's adding 8 to the temp register, not %rsp.
Duh.
