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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi,

> On 21 Oct 2020, at 10:03, Julien Grall <julien@xxxxxxx> wrote:
> 
> Hi,
> 
> On 15/10/2020 19:05, Stefano Stabellini wrote:
>> On Thu, 15 Oct 2020, Bertrand Marquis wrote:
>>>> On 14 Oct 2020, at 22:15, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
>>>> wrote:
>>>> 
>>>> On Wed, 14 Oct 2020, Julien Grall wrote:
>>>>> On 14/10/2020 17:03, Bertrand Marquis wrote:
>>>>>>> On 14 Oct 2020, at 12:35, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
>>>>>>> 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@xxxxxxx>
>>>>>>>> ---
>>>>>>>> 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.
>>>>>>> 
>>>>>>> The ARM_SMCCC_ARCH_WORKAROUND_1 instance asserts based on system state,
>>>>>>> but this provides no safety at all.
>>>>>>> 
>>>>>>> 
>>>>>>> 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.
>>>>>>> 
>>>>>>> I'm very tempted to suggest a blanket change to printk_once().
>>>>>> 
>>>>>> If this is needed then this could be done in an other serie ?
>>>>> 
>>>>> The callback ->enable() will be called when a CPU is onlined/offlined. So 
>>>>> this
>>>>> is going to require if you plan to support CPU hotplugs or suspend resume.
>>>>> 
>>>>>> Would be good to keep this patch as purely handling the errata.
>>>> 
>>>> My preference would be to keep this patch small with just the errata,
>>>> maybe using a simple printk_once as Andrew and Julien discussed.
>>>> 
>>>> There is another instance of warning_add potentially being called
>>>> outside __init in xen/arch/arm/cpuerrata.c:
>>>> enable_smccc_arch_workaround_1. So if you are up for it, it would be
>>>> good to produce a patch to fix that too.
>>>> 
>>>> 
>>>>> In the case of this patch, how about moving the warning_add() in
>>>>> enable_errata_workarounds()?
>>>>> 
>>>>> By then we should now all the errata present on your platform. All CPUs
>>>>> onlined afterwards (i.e. runtime) should always abide to the set discover
>>>>> during boot.
>>>> 
>>>> If I understand your suggestion correctly, it would work for
>>>> warn_device_load_acquire_errata, because it is just a warning, but it
>>>> would not work for enable_smccc_arch_workaround_1, because there is
>>>> actually a call to be made there.
>>>> 
>>>> Maybe it would be simpler to use printk_once in both cases? I don't have
>>>> a strong preference either way.
>>> 
>>> I could do the following (in a serie of 2 patches):
>>> - modify enable_smccc_arch_workaround_1 to use printk_once with a
>>>   prefix/suffix “****” on each line printed (and maybe adapting print to 
>>> fit a
>>>   line length of 80)
>>> - modify my patch to do the print in enable_errata_workarounds using also
>>>   the prefix/suffix and printk_once
>>> 
>>> Please confirm that this strategy would fit everyone.
>> I think it is OK but if you are going to use printk_once in your patch
>> you might as well leave it in the .enable implementation.
>> Julien, what do you think?
> 
> Bertrand reminded me today that I forgot to answer the e-mail (sorry). I am 
> happy with using printk_once().

Shall i also keep the .enable implementation ?
At the end having:
 if ( cpus_have_cap(ARM64_WORKAROUND_DEVICE_LOAD_ACQUIRE) ) 
in enable_errata_workarounds is quite clean.

> 
> I am also wondering if we should also taint the hypervisor (via add_taint()). 
> This would be helpful if someone reports error on a Xen running on such 
> platform.

Good idea yes.
I will add that and removing the core from the security supported ones to my 
patch.

Cheers
Bertrand