Re: [PATCH v4 13/17] arm64: preserve x18 when CPU is suspended

Marc Zyngier
2019-11-05T14:56:10Z

On 2019-11-05 01:11, Sami Tolvanen wrote:
> On Mon, Nov 4, 2019 at 1:59 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
>>
>> On Mon, Nov 4, 2019 at 1:38 PM Sami Tolvanen
>> <samitolvanen@google.com> wrote:
>> >
>> > On Mon, Nov 4, 2019 at 5:20 AM Marc Zyngier <maz@kernel.org>
>> wrote:
>> > > > ENTRY(cpu_do_suspend)
>> > > > mrs x2, tpidr_el0
>> > > > @@ -73,6 +75,9 @@ alternative_endif
>> > > > stp x8, x9, [x0, #48]
>> > > > stp x10, x11, [x0, #64]
>> > > > stp x12, x13, [x0, #80]
>> > > > +#ifdef CONFIG_SHADOW_CALL_STACK
>> > > > + str x18, [x0, #96]
>> > > > +#endif
>> > >
>> > > Do we need the #ifdefery here? We didn't add that to the KVM
>> path,
>> > > and I'd feel better having a single behaviour, specially when
>> > > NR_CTX_REGS is unconditionally sized to hold 13 regs.
>> >
>> > I'm fine with dropping the ifdefs here in v5 unless someone
>> objects to this.
>>
>> Oh, yeah I guess it would be good to be consistent. Rather than
>> drop
>> the ifdefs, would you (Marc) be ok with conditionally setting
>> NR_CTX_REGS based on CONFIG_SHADOW_CALL_STACK, and doing so in KVM?
>> (So 3 ifdefs, rather than 0)?
>>
>> Without any conditionals or comments, it's not clear why x18 is
>> being
>> saved and restored (unless git blame survives, or a comment is added
>> in place of the ifdefs in v6).
>
> True. Clearing the sleep state buffer in cpu_do_resume is also
> pointless without CONFIG_SHADOW_CALL_STACK, so if the ifdefs are
> removed, some kind of an explanation is needed there.

I can't imagine the overhead being noticeable, and I certainly value
minimizing the testing space. Sticking a comment there should be
enough for people hacking on this to understand that this isn't
entirely dead code.

Thanks,

M.
--
Jazz is not dead. It just smells funny...