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SubjectRe: [PATCH v2 1/2] KVM: x86: Add dedicated vCPU hint KVM_HINTS_DEDICATED
On Fri, Feb 09, 2018 at 06:47:52AM -0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
>
> Waiman Long mentioned that:
>
> Generally speaking, unfair lock performs well for VMs with a small
> number of vCPUs. Native qspinlock may perform better than pvqspinlock
> if there is vCPU pinning and there is no vCPU over-commitment.
>
> This patch adds a performance hint to allow hypervisor admin to choose
> the qspinlock to be used when a dedicated pCPU is available.
>
> PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock
> PV_DEDICATED = 0, PV_UNHALT = 1: default is Hybrid PV queued/unfair lock
> PV_DEDICATED = 0, PV_UNHALT = 0: default is tas
>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> v1 -> v2:
> * update to KVM_HINTS_DEDICATED
>
> Documentation/virtual/kvm/cpuid.txt | 6 ++++++
> arch/x86/include/uapi/asm/kvm_para.h | 2 ++
> arch/x86/kernel/kvm.c | 6 ++++++
> 3 files changed, 14 insertions(+)
>
> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> index 87a7506..d54cc79 100644
> --- a/Documentation/virtual/kvm/cpuid.txt
> +++ b/Documentation/virtual/kvm/cpuid.txt
> @@ -66,3 +66,9 @@ KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side
> || || per-cpu warps are expected in
> || || kvmclock.
> ------------------------------------------------------------------------------
> +KVM_HINTS_DEDICATED || 0 || guest checks this feature bit
> + || || to determine if they run on
> + || || dedicated vCPUs, allowing opti-
> + || || mizations such as usage of
> + || || qspinlocks.

I assume you don't mean CPUID[0x40000001].EAX[bit 0] here,
because it is already used for KVM_FEATURE_CLOCKSOURCE.

I guess you will want a separate table for CPUID[0x40000001].EDX
bits?


> +------------------------------------------------------------------------------
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 6cfa9c8..d65b16b 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -28,6 +28,8 @@
> #define KVM_FEATURE_PV_TLB_FLUSH 9
> #define KVM_FEATURE_ASYNC_PF_VMEXIT 10
>
> +#define KVM_HINTS_DEDICATED 0
> +
> /* The last 8 bits are used to indicate how to interpret the flags field
> * in pvclock structure. If no bits are set, all flags are ignored.
> */
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 971babe..7f4c92d 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -724,6 +724,12 @@ void __init kvm_spinlock_init(void)
> {
> if (!kvm_para_available())
> return;
> +
> + if (kvm_para_has_feature(KVM_HINTS_DEDICATED)) {

This is checking CPUID{0x40000001].EAX[bit 0]
(KVM_FEATURE_CLOCKSOURCE), isn't it?

> + static_branch_disable(&virt_spin_lock_key);
> + return;
> + }
> +
> /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
> if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
> return;
> --
> 2.7.4
>

--
Eduardo

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