linux-kernel - Re: [PATCH v3 04/11] kvm: arm64: Remove __hyp_this_cpu

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Date:   Fri, 18 Sep 2020 10:00:30 +0100
From:   Will Deacon <will@...nel.org>
To:     David Brazdil <dbrazdil@...gle.com>
Cc:     kvmarm@...ts.cs.columbia.edu,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, Andrew Scull <ascull@...gle.com>
Subject: Re: [PATCH v3 04/11] kvm: arm64: Remove __hyp_this_cpu_read

On Wed, Sep 16, 2020 at 06:34:32PM +0100, David Brazdil wrote:
> this_cpu_ptr is meant for use in kernel proper because it selects between
> TPIDR_EL1/2 based on nVHE/VHE. __hyp_this_cpu_ptr was used in hyp to always
> select TPIDR_EL2. Unify all users behind this_cpu_ptr and friends by
> selecting _EL2 register under __KVM_VHE/NVHE_HYPERVISOR__.
> 
> Under CONFIG_DEBUG_PREEMPT, the kernel helpers perform a preemption check
> which is omitted by the hyp helpers. Preserve the behavior for nVHE by
> overriding the corresponding macros under __KVM_NVHE_HYPERVISOR__. Extend
> the checks into VHE hyp code.
> 
> Acked-by: Andrew Scull <ascull@...gle.com>
> Signed-off-by: David Brazdil <dbrazdil@...gle.com>
> ---
>  arch/arm64/include/asm/kvm_asm.h          | 20 --------------
>  arch/arm64/include/asm/percpu.h           | 33 +++++++++++++++++++++--
>  arch/arm64/kvm/hyp/include/hyp/debug-sr.h |  4 +--
>  arch/arm64/kvm/hyp/include/hyp/switch.h   |  8 +++---
>  arch/arm64/kvm/hyp/nvhe/switch.c          |  2 +-
>  arch/arm64/kvm/hyp/vhe/switch.c           |  2 +-
>  arch/arm64/kvm/hyp/vhe/sysreg-sr.c        |  4 +--
>  7 files changed, 41 insertions(+), 32 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index c085032e2e3e..c196eec25498 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -143,26 +143,6 @@ extern char __smccc_workaround_1_smc[__SMCCC_WORKAROUND_1_SMC_SZ];
>  		addr;							\
>  	})
>  
> -/*
> - * 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
> - * a data structure. As for SHIFT_PERCPU_PTR(), the creative casting keeps
> - * sparse quiet.
> - */
> -#define __hyp_this_cpu_ptr(sym)						\
> -	({								\
> -		void *__ptr;						\
> -		__verify_pcpu_ptr(&sym);				\
> -		__ptr = hyp_symbol_addr(sym);				\
> -		__ptr += read_sysreg(tpidr_el2);			\
> -		(typeof(sym) __kernel __force *)__ptr;			\
> -	 })
> -
> -#define __hyp_this_cpu_read(sym)					\
> -	({								\
> -		*__hyp_this_cpu_ptr(sym);				\
> -	 })
> -
>  #define __KVM_EXTABLE(from, to)						\
>  	"	.pushsection	__kvm_ex_table, \"a\"\n"		\
>  	"	.align		3\n"					\
> diff --git a/arch/arm64/include/asm/percpu.h b/arch/arm64/include/asm/percpu.h
> index 0b6409b89e5e..36f304401c38 100644
> --- a/arch/arm64/include/asm/percpu.h
> +++ b/arch/arm64/include/asm/percpu.h
> @@ -19,7 +19,21 @@ static inline void set_my_cpu_offset(unsigned long off)
>  			:: "r" (off) : "memory");
>  }
>  
> -static inline unsigned long __my_cpu_offset(void)
> +static inline unsigned long __hyp_my_cpu_offset(void)
> +{
> +	unsigned long off;
> +
> +	/*
> +	 * We want to allow caching the value, so avoid using volatile and
> +	 * instead use a fake stack read to hazard against barrier().
> +	 */

I don't think we need to copy/paste the comment...

> +	asm("mrs %0, tpidr_el2" : "=r" (off) :
> +		"Q" (*(const unsigned long *)current_stack_pointer));

... especially given that we're not preemptible at EL2 with nVHE, maybe
we don't need to play this trick at all because we're always going to be
on the same CPU. So we could actually just do:

	return read_sysreg(tpidr_el2);

which is much better, and the comment should say something to that effect.

> +
> +	return off;
> +}
> +
> +static inline unsigned long __kern_my_cpu_offset(void)
>  {
>  	unsigned long off;
>  
> @@ -35,7 +49,12 @@ static inline unsigned long __my_cpu_offset(void)
>  
>  	return off;
>  }
> -#define __my_cpu_offset __my_cpu_offset()
> +
> +#if defined(__KVM_NVHE_HYPERVISOR__) || defined(__KVM_VHE_HYPERVISOR__)
> +#define __my_cpu_offset __hyp_my_cpu_offset()

Why would VHE code need to use this? Especially in light of my preemption
comments above, shouldn't it now be using __kern_my_cpu_offset()?

Will