Re: [PATCH] xen/arm: Warn user on cpu errata 832075

[Julien Grall <julien@xen.org>]

Hi Andrew,

On 14/10/2020 12:35, Andrew Cooper wrote:
> On 14/10/2020 11:41, Bertrand Marquis wrote:
>> When a Cortex A57 processor is affected by CPU errata 832075, a guest
>> not implementing the workaround for it could deadlock the system.
>> Add a warning during boot informing the user that only trusted guests
>> should be executed on the system.
>> An equivalent warning is already given to the user by KVM on cores
>> affected by this errata.
>>
>> Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
>> ---
>>   xen/arch/arm/cpuerrata.c | 21 +++++++++++++++++++++
>>   1 file changed, 21 insertions(+)
>>
>> diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
>> index 6c09017515..8f9ab6dde1 100644
>> --- a/xen/arch/arm/cpuerrata.c
>> +++ b/xen/arch/arm/cpuerrata.c
>> @@ -240,6 +240,26 @@ static int enable_ic_inv_hardening(void *data)
>>   
>>   #endif
>>   
>> +#ifdef CONFIG_ARM64_ERRATUM_832075
>> +
>> +static int warn_device_load_acquire_errata(void *data)
>> +{
>> +    static bool warned = false;
>> +
>> +    if ( !warned )
>> +    {
>> +        warning_add("This CPU is affected by the errata 832075.\n"
>> +                    "Guests without required CPU erratum workarounds\n"
>> +                    "can deadlock the system!\n"
>> +                    "Only trusted guests should be used on this system.\n");
>> +        warned = true;
> 
> This is an antipattern, which probably wants fixing elsewhere as well.
> 
> warning_add() is __init.  It's not legitimate to call from a non-init
> function, and a less useless build system would have modpost to object.

You are right. We didn't spot any issue because CPU hotplug is not yet 
supported on Arm.

> 
> The ARM_SMCCC_ARCH_WORKAROUND_1 instance asserts based on system state,
> but this provides no safety at all.

Right.

> 
> 
> What warning_add() actually does is queue messages for some point near
> the end of boot.  It's not clear that this is even a clever thing to do.

Well, the goal is to have a single place where you can find out what are 
all the inconsistencies on the platform. It can be difficult to figure 
that out with...

> 
> I'm very tempted to suggest a blanket change to printk_once().

A simple printk. But I guess we could add a wrapper that would add all a 
line of **** before and after to make easier to spot.

Cheers,

-- 
Julien Grall