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

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Date:   Tue, 05 Nov 2019 16:04:47 +0109
From:   Marc Zyngier <maz@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <dave.martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 13/17] arm64: preserve x18 when CPU is suspended

On 2019-11-05 01:11, Sami Tolvanen wrote:
> On Mon, Nov 4, 2019 at 1:59 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
>>
>> On Mon, Nov 4, 2019 at 1:38 PM Sami Tolvanen 
>> <samitolvanen@...gle.com> wrote:
>> >
>> > On Mon, Nov 4, 2019 at 5:20 AM Marc Zyngier <maz@...nel.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...