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

[Julien Grall <julien@xxxxxxx>]

Hi Bertrand,

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.
I think we should update SUPPORT.MD to say we will not security support 
those processors. Stefano, what do you think?
> An equivalent warning is already given to the user by KVM on cores
> affected by this errata.
I don't seem to find the warning in Linux. Do you have a link?

> 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;
I was going to suggest to use WARN_ON_ONCE() but it looks like it never 
made upstream :(.
> +    }
> +
> +    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
--
Julien Grall