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

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On 2019-11-05 01:11, Sami Tolvanen wrote:
On Mon, Nov 4, 2019 at 1:59 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:

On Mon, Nov 4, 2019 at 1:38 PM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
>
> On Mon, Nov 4, 2019 at 5:20 AM Marc Zyngier <maz@xxxxxxxxxx> 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...



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