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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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?