Re: [PATCH 11/13] arm64: Panic when VHE and non VHE CPUs coexist

[Will Deacon <will.deacon@arm.com>]

On Wed, Jul 08, 2015 at 05:19:14PM +0100, Marc Zyngier wrote:
> Having both VHE and non-VHE capable CPUs in the same system
> is likely to be a recipe for disaster.
> 
> If the boot CPU has VHE, but a secondary is not, we won't be
> able to downgrade and run the kernel at EL1. Add CPU hotplug
> to the mix, and this produces a terrifying mess.
> 
> Let's solve the problem once and for all. If you mix VHE and
> non-VHE CPUs in the same system, you deserve to loose, and this
> patch makes sure you don't get a chance.
> 
> This is implemented by storing the kernel execution level in
> a global variable. Secondaries will park themselves in a
> WFI loop if they observe a mismatch. Also, the primary CPU
> will detect that the secondary CPU has died on a mismatched
> execution level. Panic will follow.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>  arch/arm64/include/asm/virt.h | 15 +++++++++++++++
>  arch/arm64/kernel/head.S      | 15 +++++++++++++++
>  arch/arm64/kernel/smp.c       |  4 ++++
>  3 files changed, 34 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 9f22dd6..8e246f7 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -36,6 +36,11 @@
>   */
>  extern u32 __boot_cpu_mode[2];
>  
> +/*
> + * __run_cpu_mode records the mode the boot CPU uses for the kernel.
> + */
> +extern u32 __run_cpu_mode;
> +
>  void __hyp_set_vectors(phys_addr_t phys_vector_base);
>  phys_addr_t __hyp_get_vectors(void);
>  
> @@ -60,6 +65,16 @@ static inline bool is_kernel_in_hyp_mode(void)
>  	return el == CurrentEL_EL2;
>  }
>  
> +static inline bool is_kernel_mode_mismatched(void)
> +{
> +	u64 el;
> +	u32 mode;
> +
> +	asm("mrs %0, CurrentEL" : "=r" (el));
> +	mode = ACCESS_ONCE(__run_cpu_mode);
> +	return el != mode;

Why the temporary 'mode' variable?

> +}
> +
>  /* The section containing the hypervisor text */
>  extern char __hyp_text_start[];
>  extern char __hyp_text_end[];
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index a179747..318e69f 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -578,7 +578,20 @@ ENTRY(set_cpu_boot_mode_flag)
>  1:	str	w20, [x1]			// This CPU has booted in EL1
>  	dmb	sy
>  	dc	ivac, x1			// Invalidate potentially stale cache line
> +	adr_l	x1, __run_cpu_mode
> +	ldr	w0, [x1]
> +	mrs	x20, CurrentEL

Why x20?

> +	str	w20, [x1]
> +	dmb	sy
> +	dc	ivac, x1			// Invalidate potentially stale cache line

Can we stick __run_cpu_mode and __boot_cpu_mode into a struct in the same
cacheline and avoid the extra maintenance?

> +	cbz	x0, skip_el_check

w0?

> +	cmp	x0, x20

w0, w20?

> +	bne	mismatched_el
> +skip_el_check:
>  	ret
> +mismatched_el:
> +	wfi
> +	b	mismatched_el
>  ENDPROC(set_cpu_boot_mode_flag)
>  
>  /*
> @@ -593,6 +606,8 @@ ENDPROC(set_cpu_boot_mode_flag)
>  ENTRY(__boot_cpu_mode)
>  	.long	BOOT_CPU_MODE_EL2
>  	.long	BOOT_CPU_MODE_EL1
> +ENTRY(__run_cpu_mode)
> +	.long	0
>  	.popsection
>  
>  #ifdef CONFIG_SMP
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 695801a..b467f51 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -52,6 +52,7 @@
>  #include <asm/sections.h>
>  #include <asm/tlbflush.h>
>  #include <asm/ptrace.h>
> +#include <asm/virt.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/ipi.h>
> @@ -112,6 +113,9 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
>  			pr_crit("CPU%u: failed to come online\n", cpu);
>  			ret = -EIO;
>  		}
> +
> +		if (is_kernel_mode_mismatched())
> +			panic("CPU%u: incompatible execution level", cpu);

Might be useful to print the incompatible values, if possible.

Will