Subject: Re: [PATCH] arm64: KVM: Invoke compute_layout() before alternatives are applied

From: James Morse <[email-masked]>
Date: Thu, 17 Oct 2019 18:41:35 +0100

Hi Sebastian,

On 16/10/2019 17:59, Sebastian Andrzej Siewior wrote:
> compute_layout() is invoked as part of an alternative fixup under
> stop_machine(). This function invokes get_random_long() which acquires a
> sleeping lock on -RT which can not be acquired in this context.

> Rename compute_layout() to kvm_compute_layout() and invoke it before
> stop_machines() invokes the fixups.

Nit: stop_machine() applies the alternatives.

> Add a __init prefix to
> kvm_compute_layout() because the caller has it, too (and so the code can
> be discarded after boot).

> diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
> index b9f8d787eea9f..7532f044d713b 100644
> --- a/arch/arm64/include/asm/alternative.h
> +++ b/arch/arm64/include/asm/alternative.h
> @@ -35,6 +35,12 @@ void apply_alternatives_module(void *start, size_t length);
>  static inline void apply_alternatives_module(void *start, size_t length) { }
>  #endif
>  
> +#ifdef CONFIG_KVM_ARM_HOST
> +void kvm_compute_layout(void);
> +#else
> +static inline void kvm_compute_layout(void) { }
> +#endif

I don't think alternative.h is where this belongs... Could you move it to kvm_mmu.h, which
is where the kvm_update_va_mask macro that depends on it lives.

You can avoid the #ifdef if you use if(IS_ENABLED()) in the caller.
This has the advantage that the compiler will catch invalid C regardless of the build
options. (and its easier on the eye)

> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index d1757ef1b1e74..c28652ee06f64 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -238,6 +238,7 @@ static int __apply_alternatives_multi_stop(void *unused)
>  void __init apply_alternatives_all(void)
>  {
>  	/* better not try code patching on a live SMP system */
> +	kvm_compute_layout();
>  	stop_machine(__apply_alternatives_multi_stop, NULL, cpu_online_mask);
>  }

This is a funny place to do this kvm check, its not needed to apply the alternatives.

apply_alternatives_all() is only called from smp_cpus_done(), immediately before it calls
hyp_mode_check(), could we move it there to live with the 'started at EL2' message?

(to save you battling the header-jungle: To include asm/kvm_mmu.h, you need to include
linux/kvm_host.h first)

We end up calling it unconditionally, but I don't think that matters, both callers do the
right thing.

With that:
Reviewed-by: James Morse <[email-masked]>

Thanks,

James

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