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

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Message-ID: <20130916083221.GI17294@redhat.com>
Date:	Mon, 16 Sep 2013 11:32:22 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Marc Zyngier <marc.zyngier@....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 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?

--
			Gleb.
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