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

[Julien Grall <julien@xxxxxxx>]

Hi Bertrand,

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.
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.
Cheers,

--
Julien Grall