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

[Stefano Stabellini <sstabellini@kernel.org>]

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@citrix.com> 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.
> > > 
> > > 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.