On Mon, Jan 25, 2016 at 03:53:55PM +0000, 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 <[email protected]>
> ---
> arch/arm64/include/asm/virt.h | 17 +++++++++++++++++
> arch/arm64/kernel/head.S | 19 +++++++++++++++++++
> arch/arm64/kernel/smp.c | 3 +++
> 3 files changed, 39 insertions(+)
>
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 9f22dd6..f81a345 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[2];
> +
> void __hyp_set_vectors(phys_addr_t phys_vector_base);
> phys_addr_t __hyp_get_vectors(void);
>
> @@ -60,6 +65,18 @@ static inline bool is_kernel_in_hyp_mode(void)
> return el == CurrentEL_EL2;
> }
>
> +static inline bool is_kernel_mode_mismatched(void)
> +{
> + /*
> + * A mismatched CPU will have written its own CurrentEL in
> + * __run_cpu_mode[1] (initially set to zero) after failing to
> + * match the value in __run_cpu_mode[0]. Thus, a non-zero
> + * value in __run_cpu_mode[1] is enough to detect the
> + * pathological case.
> + */
> + return !!ACCESS_ONCE(__run_cpu_mode[1]);
> +}
> +
> /* 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 2a7134c..bc44cf8 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -577,7 +577,23 @@ 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
> + cbz x0, skip_el_check
> + cmp x0, x20
> + bne mismatched_el
can't you do a ret here instead of writing the same value and flushing
caches etc.?
> +skip_el_check: // Only the first CPU gets to set the
> rule
> + str w20, [x1]
> + dmb sy
> + dc ivac, x1 // Invalidate potentially stale cache line
> ret
> +mismatched_el:
> + str w20, [x1, #4]
> + dmb sy
> + dc ivac, x1 // Invalidate potentially stale cache line
> +1: wfi
I'm no expert on SMP bringup, but doesn't this prevent the CPU from
signaling completion and thus you'll never actually reach the checking
code in __cpu_up?
Thanks,
-Christoffer
> + b 1b
> ENDPROC(set_cpu_boot_mode_flag)
>
> /*
> @@ -592,6 +608,9 @@ 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
> + .long 0
> .popsection
>
> /*
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index b1adc51..bc7650a 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -113,6 +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);
> } else {
> pr_err("CPU%u: failed to boot: %d\n", cpu, ret);
> }
> --
> 2.1.4
>
