linux-kernel - Re: [PATCH 2/3] arm32: kvm: rename CONFIG_KVM_ARM_MAX

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Message-ID: <5236C5FF.5060301@arm.com>
Date:	Mon, 16 Sep 2013 09:49:03 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Andrew Jones <drjones@...hat.com>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"christoffer.dall@...aro.org" <christoffer.dall@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH 2/3] arm32: kvm: rename CONFIG_KVM_ARM_MAX_VCPUS

On 16/09/13 09:32, Gleb Natapov wrote:
> On Mon, Sep 16, 2013 at 09:09:27AM +0100, Marc Zyngier wrote:
>> On 15/09/13 10:30, Gleb Natapov wrote:
>>> On Sat, Sep 14, 2013 at 02:10:55PM +0200, Andrew Jones wrote:
>>>> Drop the _ARM_ part of the name. We can then introduce a config option
>>>> like this to aarch64 and other arches using the same name - allowing
>>>> grep to show them all. Also update the help text to describe the option
>>>> more completely.
>>>>
>>>> Signed-off-by: Andrew Jones <drjones@...hat.com>
>>>> ---
>>>>  arch/arm/include/asm/kvm_host.h | 4 ++--
>>>>  arch/arm/kvm/Kconfig            | 8 ++++----
>>>>  2 files changed, 6 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>>>> index 7d22517d80711..c614d3eb176c6 100644
>>>> --- a/arch/arm/include/asm/kvm_host.h
>>>> +++ b/arch/arm/include/asm/kvm_host.h
>>>> @@ -25,8 +25,8 @@
>>>>  #include <asm/fpstate.h>
>>>>  #include <kvm/arm_arch_timer.h>
>>>>  
>>>> -#if defined(CONFIG_KVM_ARM_MAX_VCPUS)
>>>> -#define KVM_MAX_VCPUS CONFIG_KVM_ARM_MAX_VCPUS
>>>> +#if defined(CONFIG_KVM_MAX_VCPUS)
>>>> +#define KVM_MAX_VCPUS CONFIG_KVM_MAX_VCPUS
>>>>  #else
>>>>  #define KVM_MAX_VCPUS 0
>>>>  #endif
>>>> diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig
>>>> index ebf5015508b52..de63bfccb3eb5 100644
>>>> --- a/arch/arm/kvm/Kconfig
>>>> +++ b/arch/arm/kvm/Kconfig
>>>> @@ -40,16 +40,16 @@ config KVM_ARM_HOST
>>>>  	---help---
>>>>  	  Provides host support for ARM processors.
>>>>  
>>>> -config KVM_ARM_MAX_VCPUS
>>>> +config KVM_MAX_VCPUS
>>>>  	int "Number maximum supported virtual CPUs per VM"
>>>>  	depends on KVM_ARM_HOST
>>>>  	default 4
>>>>  	help
>>>>  	  Static number of max supported virtual CPUs per VM.
>>>>  
>>>> -	  If you choose a high number, the vcpu structures will be quite
>>>> -	  large, so only choose a reasonable number that you expect to
>>>> -	  actually use.
>>> I do no see why on ARM vcpu structure size depends on KVM_ARM_MAX_VCPUS.
>>> Can somebody point me to it.
>>
>> There's a number of indirections, but here you go:
>> arch/arm/include/asm/kvm_host.h:#define KVM_MAX_VCPUS CONFIG_KVM_ARM_MAX_VCPUS
>>
>> include/kvm/arm_vgic.h:#define VGIC_MAX_CPUS            KVM_MAX_VCPUS
>>
>> include/kvm/arm_vgic.h:#if (VGIC_MAX_CPUS > 8)
>> include/kvm/arm_vgic.h: } percpu[VGIC_MAX_CPUS];
>> include/kvm/arm_vgic.h: u32 percpu[VGIC_MAX_CPUS][VGIC_NR_PRIVATE_IRQS / 4];
>> include/kvm/arm_vgic.h: u8                      irq_sgi_sources[VGIC_MAX_CPUS][VGIC_NR_SGIS];
>> include/kvm/arm_vgic.h: struct vgic_bitmap      irq_spi_target[VGIC_MAX_CPUS];
>>
>> Basically, the GIC emulation data structures are sized on the number of VCPUs
>> (as well as the number of supported interrupts).
>>
> But vgic is per VM though, not per cpu as far as I can see, so the
> overhead is also per VM, no? Is VGIC_MAX_CPUS limit architectural or
> juts an implementation detail?

Indeed, the MAX_VCPU overhead is per VM. The MAX_IRQS overhead is both
per-vm and per-vcpu.

The GICv2 architecture has a limit of 8 CPUs - not at all an
implementation detail. GICv3 remove this limitation.

	M.
-- 
Jazz is not dead. It just smells funny...

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