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

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Date:   Mon, 21 Oct 2019 08:19:23 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.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>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/18] arm64: kvm: stop treating register x18 as caller save

On Fri, 18 Oct 2019 at 18:10, Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>
> In preparation of using x18 as a task struct pointer register when
> running in the kernel, stop treating it as caller save in the KVM
> guest entry/exit code. Currently, the code assumes there is no need
> to preserve it for the host, given that it would have been assumed
> clobbered anyway by the function call to __guest_enter(). Instead,
> preserve its value and restore it upon return.
>
> Link: https://patchwork.kernel.org/patch/9836891/
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

You might want to update the commit log to drop the reference to the
task struct pointer.

> ---
>  arch/arm64/kvm/hyp/entry.S | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index e5cc8d66bf53..20bd9a20ea27 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -23,6 +23,7 @@
>         .pushsection    .hyp.text, "ax"
>
>  .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)]
>  .endm
>
>  /*
> @@ -87,12 +89,9 @@ alternative_else_nop_endif
>         ldp     x14, x15, [x18, #CPU_XREG_OFFSET(14)]
>         ldp     x16, x17, [x18, #CPU_XREG_OFFSET(16)]
>
> -       // Restore guest regs x19-x29, lr
> +       // Restore guest regs x18-x29, lr
>         restore_callee_saved_regs x18
>
> -       // Restore guest reg x18
> -       ldr     x18,      [x18, #CPU_XREG_OFFSET(18)]
> -
>         // Do not touch any register after this!
>         eret
>         sb
> @@ -114,7 +113,7 @@ ENTRY(__guest_exit)
>         // Retrieve the guest regs x0-x1 from the stack
>         ldp     x2, x3, [sp], #16       // x0, x1
>
> -       // Store the guest regs x0-x1 and x4-x18
> +       // Store the guest regs x0-x1 and x4-x17
>         stp     x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
>         stp     x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
>         stp     x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
> @@ -123,9 +122,8 @@ ENTRY(__guest_exit)
>         stp     x12, x13, [x1, #CPU_XREG_OFFSET(12)]
>         stp     x14, x15, [x1, #CPU_XREG_OFFSET(14)]
>         stp     x16, x17, [x1, #CPU_XREG_OFFSET(16)]
> -       str     x18,      [x1, #CPU_XREG_OFFSET(18)]
>
> -       // Store the guest regs x19-x29, lr
> +       // Store the guest regs x18-x29, lr
>         save_callee_saved_regs x1
>
>         get_host_ctxt   x2, x3
> --
> 2.23.0.866.gb869b98d4c-goog
>