[PATCH] ARM/ARM64: KVM: remove 'config KVM_ARM_MAX_VCPUS'
Ming Lei
ming.lei at canonical.com
Wed Sep 2 04:42:08 PDT 2015
On Wed, Sep 2, 2015 at 6:25 PM, Christoffer Dall
<christoffer.dall at linaro.org> wrote:
> On Wed, Sep 02, 2015 at 02:31:21PM +0800, Ming Lei wrote:
>> This patch removes config option of KVM_ARM_MAX_VCPUS,
>> and like other ARCHs, just choose the maximum allowed
>> value from hardware, and follows the reasons:
>>
>> 1) from distribution view, the option has to be
>> defined as the max allowed value because it need to
>> meet all kinds of virtulization applications and
>> need to support most of SoCs;
>>
>> 2) using a bigger value doesn't introduce extra memory
>> consumption, and the help text in Kconfig isn't accurate
>> because kvm_vpu structure isn't allocated until request
>> of creating VCPU is sent from QEMU;
>
> This used to be true because of the vgic bitmaps, but that is now
> dynamically allocated, so I believe you're correct in saying that the
> text is no longer accurate.
>
>>
>> 3) the main effect is that the field of vcpus[] in 'struct kvm'
>> becomes a bit bigger(sizeof(void *) per vcpu) and need more cache
>> lines to hold the structure, but 'struct kvm' is one generic struct,
>> and it has worked well on other ARCHs already in this way. Also,
>> the world switch frequecy is often low, for example, it is ~2000
>> when running kernel building load in VM from APM xgene KVM host,
>> so the effect is very small, and the difference can't be observed
>> in my test at all.
>
> While I'm not prinicipally opposed to removing this option, I have to
> point out that this analysis is far far over-simplified. You have
> chosen a workload which excercised only CPU and memory virtualization,
> mostly solved by the hardware virtualization support, and therefore you
> don't see many exits.
>
> Try running an I/O bound workload, or something which involves a lot of
> virtual IPIs, and you'll see a higher number of exits.
Yeah, the frequency of exits becomes higher(6600/sec) when I run a
totally I/O benchmark(fio: 4 jobs, bs 4k, libaio over virtio-blk) in a
quad-core VM, but it is still not high enough to cause any difference
on the test result.
>
> However, I still doubt that the effects will be noticable in the grand
> scheme of things.
>>
>> Cc: Dann Frazier <dann.frazier at canonical.com>
>> Cc: Christoffer Dall <christoffer.dall at linaro.org>
>> Cc: Marc Zyngier <marc.zyngier at arm.com>
>> Cc: kvmarm at lists.cs.columbia.edu
>> Cc: kvm at vger.kernel.org
>> Signed-off-by: Ming Lei <ming.lei at canonical.com>
>> ---
>> arch/arm/include/asm/kvm_host.h | 8 ++------
>> arch/arm/kvm/Kconfig | 11 -----------
>> arch/arm64/include/asm/kvm_host.h | 8 ++------
>> arch/arm64/kvm/Kconfig | 11 -----------
>> include/kvm/arm_vgic.h | 6 +-----
>> virt/kvm/arm/vgic-v3.c | 2 +-
>> 6 files changed, 6 insertions(+), 40 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index dcba0fa..c8c226a 100644
>> --- a/arch/arm/include/asm/kvm_host.h
>> +++ b/arch/arm/include/asm/kvm_host.h
>> @@ -29,12 +29,6 @@
>>
>> #define __KVM_HAVE_ARCH_INTC_INITIALIZED
>>
>> -#if defined(CONFIG_KVM_ARM_MAX_VCPUS)
>> -#define KVM_MAX_VCPUS CONFIG_KVM_ARM_MAX_VCPUS
>> -#else
>> -#define KVM_MAX_VCPUS 0
>> -#endif
>> -
>> #define KVM_USER_MEM_SLOTS 32
>> #define KVM_PRIVATE_MEM_SLOTS 4
>> #define KVM_COALESCED_MMIO_PAGE_OFFSET 1
>> @@ -44,6 +38,8 @@
>>
>> #include <kvm/arm_vgic.h>
>>
>> +#define KVM_MAX_VCPUS VGIC_V2_MAX_CPUS
>> +
>> u32 *kvm_vcpu_reg(struct kvm_vcpu *vcpu, u8 reg_num, u32 mode);
>> int __attribute_const__ kvm_target_cpu(void);
>> int kvm_reset_vcpu(struct kvm_vcpu *vcpu);
>> diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig
>> index bfb915d..210ecca 100644
>> --- a/arch/arm/kvm/Kconfig
>> +++ b/arch/arm/kvm/Kconfig
>> @@ -45,15 +45,4 @@ config KVM_ARM_HOST
>> ---help---
>> Provides host support for ARM processors.
>>
>> -config KVM_ARM_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.
>> -
>> endif # VIRTUALIZATION
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 415938d..3fb58ea 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -30,12 +30,6 @@
>>
>> #define __KVM_HAVE_ARCH_INTC_INITIALIZED
>>
>> -#if defined(CONFIG_KVM_ARM_MAX_VCPUS)
>> -#define KVM_MAX_VCPUS CONFIG_KVM_ARM_MAX_VCPUS
>> -#else
>> -#define KVM_MAX_VCPUS 0
>> -#endif
>> -
>> #define KVM_USER_MEM_SLOTS 32
>> #define KVM_PRIVATE_MEM_SLOTS 4
>> #define KVM_COALESCED_MMIO_PAGE_OFFSET 1
>> @@ -43,6 +37,8 @@
>> #include <kvm/arm_vgic.h>
>> #include <kvm/arm_arch_timer.h>
>>
>> +#define KVM_MAX_VCPUS VGIC_V3_MAX_CPUS
>> +
>> #define KVM_VCPU_MAX_FEATURES 3
>>
>> int __attribute_const__ kvm_target_cpu(void);
>> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
>> index bfffe8f..5c7e920 100644
>> --- a/arch/arm64/kvm/Kconfig
>> +++ b/arch/arm64/kvm/Kconfig
>> @@ -41,15 +41,4 @@ config KVM_ARM_HOST
>> ---help---
>> Provides host support for ARM processors.
>>
>> -config KVM_ARM_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.
>> -
>> endif # VIRTUALIZATION
>> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
>> index d901f1a..4e14dac 100644
>> --- a/include/kvm/arm_vgic.h
>> +++ b/include/kvm/arm_vgic.h
>> @@ -35,11 +35,7 @@
>> #define VGIC_V3_MAX_LRS 16
>> #define VGIC_MAX_IRQS 1024
>> #define VGIC_V2_MAX_CPUS 8
>> -
>> -/* Sanity checks... */
>> -#if (KVM_MAX_VCPUS > 255)
>> -#error Too many KVM VCPUs, the VGIC only supports up to 255 VCPUs for now
>> -#endif
>> +#define VGIC_V3_MAX_CPUS 255
>>
>> #if (VGIC_NR_IRQS_LEGACY & 31)
>> #error "VGIC_NR_IRQS must be a multiple of 32"
>> diff --git a/virt/kvm/arm/vgic-v3.c b/virt/kvm/arm/vgic-v3.c
>> index afbf925..7dd5d62 100644
>> --- a/virt/kvm/arm/vgic-v3.c
>> +++ b/virt/kvm/arm/vgic-v3.c
>> @@ -288,7 +288,7 @@ int vgic_v3_probe(struct device_node *vgic_node,
>>
>> vgic->vctrl_base = NULL;
>> vgic->type = VGIC_V3;
>> - vgic->max_gic_vcpus = KVM_MAX_VCPUS;
>> + vgic->max_gic_vcpus = VGIC_V3_MAX_CPUS;
>>
>> kvm_info("%s@%llx IRQ%d\n", vgic_node->name,
>> vcpu_res.start, vgic->maint_irq);
>> --
>> 1.9.1
>
>
> Reviewed-by: Christoffer Dall <christoffer.dall at linaro.org>
Thanks for your review.
Thanks,
Ming
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