Re: [PATCH] KVM: arm64: Stop sparse from moaning at __hyp_this_cpu_ptr

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On Mon, Jun 08, 2020 at 09:57:31AM +0100, Marc Zyngier wrote:
> Sparse complains that __hyp_this_cpu_ptr() returns something
> that is flagged noderef and not in the correct address space
> (both being the result of the __percpu annotation).
> 
> Pretend that __hyp_this_cpu_ptr() knows what it is doing by
> forcefully casting the pointer with __kernel __force.
> 
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_asm.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 0c9b5fc4ba0a..82691406d493 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -81,12 +81,17 @@ extern u32 __kvm_get_mdcr_el2(void);
>  
>  extern char __smccc_workaround_1_smc[__SMCCC_WORKAROUND_1_SMC_SZ];
>  
> -/* Home-grown __this_cpu_{ptr,read} variants that always work at HYP */
> +/*
> + * Home-grown __this_cpu_{ptr,read} variants that always work at HYP,
> + * provided that sym is really a *symbol* and not a pointer obtained from

Look at `this_cpu_ptr` one thing that stood out was `__verify_pcpu_ptr`
that is documented to be suitable for used in custom per CPU macros. I
didn't get how it worked (a type check?) but maybe it would work here
to validate the argment was indeed a per CPU symbol?

> + * a data structure. As for SHIFT_PERCPU_PTR(), the creative casting keeps
> + * sparse quiet.
> + */
>  #define __hyp_this_cpu_ptr(sym)						\
>  	({								\
>  		void *__ptr = hyp_symbol_addr(sym);			\
>  		__ptr += read_sysreg(tpidr_el2);			\
> -		(typeof(&sym))__ptr;					\
> +		(typeof(sym) __kernel __force *)__ptr;			\
>  	 })
>  
>  #define __hyp_this_cpu_read(sym)					\



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