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
>>> 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.