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

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Message-ID: <20191022182206.0d8b2301@why>
Date:   Tue, 22 Oct 2019 18:22:06 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
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@...ts.openwall.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Laura Abbott <labbott@...hat.com>,
        Dave Martin <Dave.Martin@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 03/18] arm64: kvm: stop treating register x18 as caller
 save

On Fri, 18 Oct 2019 09:10:18 -0700
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>
> ---
>  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)]

There is now an assumption that ctxt is x18 (otherwise why would it be
out of order?). Please add a comment to that effect.

>  .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

Or you could elect another register such as x29 as the base, and keep
the above in a reasonable order.

>  
> -	// 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

Thanks,

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