linux-kernel - Re: [PATCH v3 56/59] KVM: arm/arm64: GICv4: Prevent heterogenous systems from using GICv4

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Date:   Mon, 28 Aug 2017 20:35:50 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Eric Auger <eric.auger@...hat.com>,
        Shanker Donthineni <shankerd@...eaurora.org>,
        Mark Rutland <mark.rutland@....com>,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>
Subject: Re: [PATCH v3 56/59] KVM: arm/arm64: GICv4: Prevent heterogenous
 systems from using GICv4

On Mon, Jul 31, 2017 at 06:26:34PM +0100, Marc Zyngier wrote:
> The GICv4 architecture doesn't prevent CPUs implementing GICv4 to
> cohabit with CPUs limited to GICv3 in the same system.
> 
> This is mad (the sheduler would have to be made aware of the v4

*scheduler

> capability), and we're certainly not going to support this any
> time soon. So let's check that all online CPUs are GICv4 capable,
> and disable the functionnality if not.

*functionality

> 
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> ---
>  include/linux/irqchip/arm-gic-v3.h |  2 ++
>  virt/kvm/arm/vgic/vgic-v3.c        | 22 +++++++++++++++++++++-
>  2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/irqchip/arm-gic-v3.h b/include/linux/irqchip/arm-gic-v3.h
> index 1ea576c8126f..dfa4a51643d6 100644
> --- a/include/linux/irqchip/arm-gic-v3.h
> +++ b/include/linux/irqchip/arm-gic-v3.h
> @@ -532,6 +532,8 @@
>  #define ICH_VTR_SEIS_MASK		(1 << ICH_VTR_SEIS_SHIFT)
>  #define ICH_VTR_A3V_SHIFT		21
>  #define ICH_VTR_A3V_MASK		(1 << ICH_VTR_A3V_SHIFT)
> +#define ICH_VTR_nV4_SHIFT		20
> +#define ICH_VTR_nV4_MASK		(1 << ICH_VTR_nV4_SHIFT)
>  
>  #define ICC_IAR1_EL1_SPURIOUS		0x3ff
>  
> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
> index 405733678c2f..252268e83ade 100644
> --- a/virt/kvm/arm/vgic/vgic-v3.c
> +++ b/virt/kvm/arm/vgic/vgic-v3.c
> @@ -466,6 +466,18 @@ static int __init early_gicv4_enable(char *buf)
>  }
>  early_param("kvm-arm.vgic_v4_enable", early_gicv4_enable);
>  
> +static void vgic_check_v4_cpuif(void *param)
> +{
> +	u32 ich_vtr_el2 = kvm_call_hyp(__vgic_v3_get_ich_vtr_el2);
> +	bool *v4 = param, this_cpu_is_v4;
> +
> +	this_cpu_is_v4 = !(ich_vtr_el2 & ICH_VTR_nV4_MASK);
> +	if (!this_cpu_is_v4)
> +		kvm_info("CPU%d is not GICv4 capable\n", smp_processor_id());
> +
> +	*v4 &= this_cpu_is_v4;

nit: You could make this function look slightly less scary by declaring
this_cpu_is_v4 on a separate line and not use a bitwise operator on a
boolean type.  Actually, having a function called 'check' with a side
effect is not the nicest thing either, so why not just return what this
particular CPU does and do the comparison in the calling loop.

#bikeshedding


Thanks,
-Christoffer



> +}
> +
>  /**
>   * vgic_v3_probe - probe for a GICv3 compatible interrupt controller in DT
>   * @node:	pointer to the DT node
> @@ -485,8 +497,16 @@ int vgic_v3_probe(const struct gic_kvm_info *info)
>  	kvm_vgic_global_state.can_emulate_gicv2 = false;
>  	kvm_vgic_global_state.ich_vtr_el2 = ich_vtr_el2;
>  
> -	/* GICv4 support? */
> +	/*
> +	 * GICv4 support? We need to check on all CPUs in case of some
> +	 * extremely creative form of big-little brain damage...
> +	 */
>  	if (info->has_v4) {
> +		int cpu;
> +
> +		for_each_online_cpu(cpu)
> +			smp_call_function_single(cpu, vgic_check_v4_cpuif,
> +						 &gicv4_enable, 1);
>  		kvm_vgic_global_state.has_gicv4 = gicv4_enable;
>  		kvm_info("GICv4 support %sabled\n",
>  			 gicv4_enable ? "en" : "dis");
> -- 
> 2.11.0
>