Re: [PATCH v2 21/21] arm64: Panic when VHE and non VHE CPUs coexist

[Marc Zyngier]

On 03/02/16 08:49, Christoffer Dall wrote:
> On Tue, Feb 02, 2016 at 03:32:04PM +0000, Marc Zyngier wrote:
>> On 01/02/16 15:36, Christoffer Dall wrote:
>>> 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 <marc.zyngier@xxxxxxx>
>>>> ---
>>>>  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.?
>>
>> Yes, good point.
>>
>>>
>>>> +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?
>>
>> Indeed, and that's the whole point. The primary CPU will notice that the
>> secondary CPU has failed to boot (timeout), and will find the reason in
>> __run_cpu_mode.
>>
> That wasn't exactly my point.  If I understand correctly and __cpu_up is
> the primary CPU executing a function to bring up a secondary core, then
> it will wait for the cpu_running completion which should be signalled by
> the secondary core, but because the secondary core never makes any
> progress it will timeout the wait for completion and you will see that
> error "..failed to come online" instead of the "incompatible execution
> level".

It will actually do both. Here's an example on the model configured for
such a braindead case:

CPU4: failed to come online
Kernel panic - not syncing: CPU4: incompatible execution level
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.5.0-rc2+ #5459
Hardware name: FVP Base (DT)
Call trace:
[<ffffffc0000899e0>] dump_backtrace+0x0/0x180
[<ffffffc000089b74>] show_stack+0x14/0x20
[<ffffffc000333b08>] dump_stack+0x90/0xc8
[<ffffffc00014d424>] panic+0x10c/0x250
[<ffffffc00008ef24>] __cpu_up+0xfc/0x100
[<ffffffc0000b7a9c>] _cpu_up+0x154/0x188
[<ffffffc0000b7b54>] cpu_up+0x84/0xa8
[<ffffffc0009e9d00>] smp_init+0xbc/0xc0
[<ffffffc0009dca10>] kernel_init_freeable+0x94/0x1ec
[<ffffffc000712f90>] kernel_init+0x10/0xe0
[<ffffffc000085cd0>] ret_from_fork+0x10/0x40

Am I missing something *really* obvious?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...