Subject: Re: [PATCH 1/3] arm/arm64/xen: use C inlines for privcmd_call

From: Russell King - ARM Linux admin <[email-masked]>
Date: Fri, 22 Nov 2019 00:22:58 +0000

On Thu, Nov 21, 2019 at 01:48:03PM -0500, Pavel Tatashin wrote:
> privcmd_call requires to enable access to userspace for the
> duration of the hypercall.
> 
> Currently, this is done via assembly macros. Change it to C
> inlines instead.
> 
> Signed-off-by: Pavel Tatashin <[email-masked]>
> ---
>  arch/arm/include/asm/assembler.h |  2 +-
>  arch/arm/include/asm/uaccess.h   | 32 ++++++++++++++++++++++++++------
>  arch/arm/xen/enlighten.c         |  2 +-
>  arch/arm/xen/hypercall.S         | 15 ++-------------
>  arch/arm64/xen/hypercall.S       | 19 ++-----------------
>  include/xen/arm/hypercall.h      | 23 ++++++++++++++++++++---
>  6 files changed, 52 insertions(+), 41 deletions(-)
> 
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index 99929122dad7..8e9262a0f016 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -480,7 +480,7 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
>  	.macro	uaccess_disable, tmp, isb=1
>  #ifdef CONFIG_CPU_SW_DOMAIN_PAN
>  	/*
> -	 * Whenever we re-enter userspace, the domains should always be
> +	 * Whenever we re-enter kernel, the domains should always be
>  	 * set appropriately.
>  	 */
>  	mov	\tmp, #DACR_UACCESS_DISABLE
> diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
> index 98c6b91be4a8..79d4efa3eb62 100644
> --- a/arch/arm/include/asm/uaccess.h
> +++ b/arch/arm/include/asm/uaccess.h
> @@ -16,6 +16,23 @@
>  
>  #include <asm/extable.h>
>  
> +#ifdef CONFIG_CPU_SW_DOMAIN_PAN
> +static __always_inline void uaccess_enable(void)
> +{
> +	unsigned long val = DACR_UACCESS_ENABLE;
> +
> +	asm volatile("mcr p15, 0, %0, c3, c0, 0" : : "r" (val));
> +	isb();
> +}
> +
> +static __always_inline void uaccess_disable(void)
> +{
> +	unsigned long val = DACR_UACCESS_ENABLE;
> +
> +	asm volatile("mcr p15, 0, %0, c3, c0, 0" : : "r" (val));
> +	isb();
> +}

Rather than inventing these, why not use uaccess_save_and_enable()..
uaccess_restore() around the Xen call?

-- 
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