linux/arch/arm/kvm/Kconfig
Marc Zyngier 58d5ec8f8e ARM: KVM: Yield CPU when vcpu executes a WFE
On an (even slightly) oversubscribed system, spinlocks are quickly
becoming a bottleneck, as some vcpus are spinning, waiting for a
lock to be released, while the vcpu holding the lock may not be
running at all.

This creates contention, and the observed slowdown is 40x for
hackbench. No, this isn't a typo.

The solution is to trap blocking WFEs and tell KVM that we're
now spinning. This ensures that other vpus will get a scheduling
boost, allowing the lock to be released more quickly. Also, using
CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT slightly improves the performance
when the VM is severely overcommited.

Quick test to estimate the performance: hackbench 1 process 1000

2xA15 host (baseline):	1.843s

2xA15 guest w/o patch:	2.083s
4xA15 guest w/o patch:	80.212s
8xA15 guest w/o patch:	Could not be bothered to find out

2xA15 guest w/ patch:	2.102s
4xA15 guest w/ patch:	3.205s
8xA15 guest w/ patch:	6.887s

So we go from a 40x degradation to 1.5x in the 2x overcommit case,
which is vaguely more acceptable.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
2013-10-17 15:26:50 -07:00

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#
# KVM configuration
#
source "virt/kvm/Kconfig"
menuconfig VIRTUALIZATION
bool "Virtualization"
---help---
Say Y here to get to see options for using your Linux host to run
other operating systems inside virtual machines (guests).
This option alone does not add any kernel code.
If you say N, all options in this submenu will be skipped and
disabled.
if VIRTUALIZATION
config KVM
bool "Kernel-based Virtual Machine (KVM) support"
select PREEMPT_NOTIFIERS
select ANON_INODES
select HAVE_KVM_CPU_RELAX_INTERCEPT
select KVM_MMIO
select KVM_ARM_HOST
depends on ARM_VIRT_EXT && ARM_LPAE
---help---
Support hosting virtualized guest machines. You will also
need to select one or more of the processor modules below.
This module provides access to the hardware capabilities through
a character device node named /dev/kvm.
If unsure, say N.
config KVM_ARM_HOST
bool "KVM host support for ARM cpus."
depends on KVM
depends on MMU
select MMU_NOTIFIER
---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.
config KVM_ARM_VGIC
bool "KVM support for Virtual GIC"
depends on KVM_ARM_HOST && OF
select HAVE_KVM_IRQCHIP
default y
---help---
Adds support for a hardware assisted, in-kernel GIC emulation.
config KVM_ARM_TIMER
bool "KVM support for Architected Timers"
depends on KVM_ARM_VGIC && ARM_ARCH_TIMER
select HAVE_KVM_IRQCHIP
default y
---help---
Adds support for the Architected Timers in virtual machines
endif # VIRTUALIZATION