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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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().

~Andrew

> +    }
> +
> +    return 0;
> +}
> +
> +#endif
> +
>  #ifdef CONFIG_ARM_SSBD
>  
>  enum ssbd_state ssbd_state = ARM_SSBD_RUNTIME;
> @@ -419,6 +439,7 @@ static const struct arm_cpu_capabilities arm_errata[] = {
>          .capability = ARM64_WORKAROUND_DEVICE_LOAD_ACQUIRE,
>          MIDR_RANGE(MIDR_CORTEX_A57, 0x00,
>                     (1 << MIDR_VARIANT_SHIFT) | 2),
> +        .enable = warn_device_load_acquire_errata,
>      },
>  #endif
>  #ifdef CONFIG_ARM64_ERRATUM_834220