Re: [Xen-devel] [PATCH v2] xen/arm: Park CPUs with a MIDR different from the boot CPU.

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

Hi, Julien.

On Wed, Feb 14, 2018 at 4:17 PM, Julien Grall <julien.grall@xxxxxxx> 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.
>
>     Changes in v2:
>         - Add a command line option to override the default behavior.
> ---
>  docs/misc/xen-command-line.markdown | 10 ++++++++++
>  xen/arch/arm/smpboot.c              | 26 ++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+)
>
> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
> index 79feba6bcd..cf5997b8db 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -1000,6 +1000,16 @@ supported only when compiled with XSM on x86.
>
>  Control Xens use of the APEI Hardware Error Source Table, should one be 
> found.
>
> +### hmp_unsafe (arm)
> +> `= <boolean>`
> +
> +> Default : `false`
> +
> +Say yes at your own risk if you want to enable heterogenous computing
> +(such as big.LITTLE). This may result to an unstable and insecure
> +platform. When the option is disabled (default), CPUs that are not
> +identical to the boot CPU will be parked and not used by Xen.
> +
>  ### hpetbroadcast
>  > `= <boolean>`
>
> diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
> index 1255185a9c..5c05cadb0a 100644
> --- a/xen/arch/arm/smpboot.c
> +++ b/xen/arch/arm/smpboot.c
> @@ -27,6 +27,7 @@
>  #include <xen/smp.h>
>  #include <xen/softirq.h>
>  #include <xen/timer.h>
> +#include <xen/warning.h>
>  #include <xen/irq.h>
>  #include <xen/console.h>
>  #include <asm/cpuerrata.h>
> @@ -69,6 +70,13 @@ DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, 
> cpu_sibling_mask);
>  /* representing HT and core siblings of each logical CPU */
>  DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_core_mask);
>
> +/*
> + * By default non-boot CPUs not identical to the boot CPU will be
> + * parked.
> + */
> +static bool __read_mostly opt_hmp_unsafe = false;
> +boolean_param("hmp_unsafe", opt_hmp_unsafe);
> +
>  static void setup_cpu_sibling_map(int cpu)
>  {
>      if ( !zalloc_cpumask_var(&per_cpu(cpu_sibling_mask, cpu)) ||
> @@ -255,6 +263,9 @@ void __init smp_init_cpus(void)
>      else
>          acpi_smp_init_cpus();
>
> +    if ( opt_hmp_unsafe )
> +        warning_add("WARNING: HMP COMPUTING HAS BEEN ENABLED.\n"
> +                    "It has implications on the security and stability of 
> the system.\n");
>  }
>
>  int __init
> @@ -292,6 +303,21 @@ 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 instability and insecure platform. Better to park them
> +     * for now.
> +     */
> +    if ( !opt_hmp_unsafe &&
> +         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();
> --
> 2.11.0
>
>
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Thanks for leaving an option to enable hmp back. We understand the
possible risks, but we are playing with big and LITTLE cores for test
purposes
and without such option we will have to revert the patch to allow
LITTLE cores to up and running.

-- 
Regards,

Oleksandr Tyshchenko

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