Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 15 Feb 2018 00:17:46 +0000 | Subject | Re: [RFC PATCH 2/4] x86/entry/64: move ENTER_IRQ_STACK from interrupt macro to helper function |
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On Wed, Feb 14, 2018 at 6:21 PM, Dominik Brodowski <linux@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 <linux@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?
--Andy
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