linux-kernel - Re: [PATCH 03/18] arm64: kvm: stop treating register x18 as caller save

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Date:   Tue, 22 Oct 2019 14:45:45 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Laura Abbott <labbott@...hat.com>,
        Dave Martin <Dave.Martin@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 03/18] arm64: kvm: stop treating register x18 as caller save

On Tue, Oct 22, 2019 at 10:22 AM Marc Zyngier <maz@...nel.org> wrote:
> >  .macro save_callee_saved_regs ctxt
> > +     str     x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
> >       stp     x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
> >       stp     x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
> >       stp     x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
> > @@ -38,6 +39,7 @@
> >       ldp     x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
> >       ldp     x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
> >       ldp     x29, lr,  [\ctxt, #CPU_XREG_OFFSET(29)]
> > +     ldr     x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
>
> There is now an assumption that ctxt is x18 (otherwise why would it be
> out of order?). Please add a comment to that effect.

> > -     // Restore guest regs x19-x29, lr
> > +     // Restore guest regs x18-x29, lr
> >       restore_callee_saved_regs x18
>
> Or you could elect another register such as x29 as the base, and keep
> the above in a reasonable order.

I'm fine with either option. Ard, any thoughts?

Sami