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

[Julien Grall <julien@xxxxxxx>]

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@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.
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