Re: [Xen-devel] [PATCH 8/9] xen/arm: Park CPUs with a MIDR different from the boot CPU.

[Julien Grall <julien.grall@xxxxxxx>]

Hmmm sorry, I was not meant to be sent with this series. It is also a 
duplicate of the same patch sent last week.
On 05/02/18 13:20, Julien Grall wrote:
> Xen does not properly support big.LITTLE platform. All vCPUs of a guest
> will always have the MIDR of the boot CPU (see arch_domain_create).
> At best the guest may see unreliable performance (vCPU switching between
> big and LITTLE), at worst the guest will become unreliable or insecure.
>
> This is becoming more apparent with branch predictor hardening in Linux
> because they target a specific kind of CPUs and may not work on other
> CPUs.
>
> For the time being, park any CPUs with a MDIR different from the boot
> CPU. This will be revisited in the future once Xen gains understanding
> of big.LITTLE.
>
> [1] https://lists.xenproject.org/archives/html/xen-devel/2016-12/msg00826.html
>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
>
> ---
>
> We probably want to backport this as part of XSA-254. Using big.LITTLE
> on Xen has never been supported but we didn't make it clearly. This is
> becoming more apparent with code targeting specific CPUs.
> ---
>   xen/arch/arm/smpboot.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
>
> diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
> index 1255185a9c..2c2815f9ee 100644
> --- a/xen/arch/arm/smpboot.c
> +++ b/xen/arch/arm/smpboot.c
> @@ -292,6 +292,21 @@ void start_secondary(unsigned long boot_phys_offset,
>   
>       init_traps();
>   
> +    /*
> +     * Currently Xen assumes the platform has only one kind of CPUs.
> +     * This assumption does not hold on big.LITTLE platform and may
> +     * result to unstability. Better to park them for now.
> +     *
> +     * TODO: Add big.LITTLE support.
> +     */
> +    if ( current_cpu_data.midr.bits != boot_cpu_data.midr.bits )
> +    {
> +        printk(XENLOG_ERR "CPU%u MIDR (0x%x) does not match boot CPU MIDR 
> (0x%x).\n",
> +               smp_processor_id(), current_cpu_data.midr.bits,
> +               boot_cpu_data.midr.bits);
> +        stop_cpu();
> +    }
> +
>       mmu_init_secondary_cpu();
>   
>       gic_init_secondary_cpu();
--
Julien Grall

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