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Date:   Thu, 9 Nov 2017 20:43:02 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Eduardo Valentin <eduval@...zon.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Matt Wilson <msw@...zon.com>, Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>, kvm <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jan H . Schoenherr" <jschoenh@...zon.de>,
        Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

2017-11-07 4:26 GMT+08:00 Eduardo Valentin <eduval@...zon.com>:
> Currently, the existing qspinlock implementation will fallback to
> test-and-set if the hypervisor has not set the PV_UNHALT flag.
>
> This patch gives the opportunity to guest kernels to select
> between test-and-set and the regular queueu fair lock implementation
> based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED
> flag is not set, the code will still fall back to test-and-set,
> but when the PV_DEDICATED flag is set, the code will use
> the regular queue spinlock implementation.
>
> With this patch, when in autoselect mode, the guest will
> use the default spinlock implementation based on host feature
> flags as follows:
>
> PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock
> PV_DEDICATED = 0, PV_UNHALT = 1: default is pvqspinlock
> PV_DEDICATED = 0, PV_UNHALT = 0: default is tas
>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: kvm@...r.kernel.org
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: Jan H. Schoenherr <jschoenh@...zon.de>
> Cc: Anthony Liguori <aliguori@...zon.com>
> Suggested-by: Matt Wilson <msw@...zon.com>
> Signed-off-by: Eduardo Valentin <eduval@...zon.com>
> ---
> V3:
>  - When PV_DEDICATED is set (1), qspinlock is selected,
>    regardless of the value of PV_UNHAULT. Suggested by Paolo Bonzini.
>  - Refreshed on top of tip/master.
> V2:
>  - rebase on top of tip/master
>
>  Documentation/virtual/kvm/cpuid.txt  | 6 ++++++
>  arch/x86/include/asm/qspinlock.h     | 4 ++++
>  arch/x86/include/uapi/asm/kvm_para.h | 1 +
>  arch/x86/kernel/kvm.c                | 2 ++
>  4 files changed, 13 insertions(+)
>
> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> index 3c65feb..117066a 100644
> --- a/Documentation/virtual/kvm/cpuid.txt
> +++ b/Documentation/virtual/kvm/cpuid.txt
> @@ -54,6 +54,12 @@ KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
>                                     ||       || before enabling paravirtualized
>                                     ||       || spinlock support.
>  ------------------------------------------------------------------------------
> +KVM_FEATURE_PV_DEDICATED           ||     8 || guest checks this feature bit
> +                                   ||       || to determine if they run on
> +                                   ||       || dedicated vCPUs, allowing opti-
> +                                   ||       || mizations such as usage of
> +                                   ||       || qspinlocks.
> +------------------------------------------------------------------------------
>  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
>                                     ||       || per-cpu warps are expected in
>                                     ||       || kvmclock.
> diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
> index 5e16b5d..de42694 100644
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -3,6 +3,8 @@
>  #define _ASM_X86_QSPINLOCK_H
>
>  #include <linux/jump_label.h>
> +#include <linux/kvm_para.h>
> +
>  #include <asm/cpufeature.h>
>  #include <asm-generic/qspinlock_types.h>
>  #include <asm/paravirt.h>
> @@ -58,6 +60,8 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
>         if (!static_branch_likely(&virt_spin_lock_key))
>                 return false;
>
> +       if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED))
> +               return false;
>         /*
>          * On hypervisors without PARAVIRT_SPINLOCKS support we fall
>          * back to a Test-and-Set spinlock, because fair locks have
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 554aa8f..85a9875 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -25,6 +25,7 @@
>  #define KVM_FEATURE_STEAL_TIME         5
>  #define KVM_FEATURE_PV_EOI             6
>  #define KVM_FEATURE_PV_UNHALT          7
> +#define KVM_FEATURE_PV_DEDICATED       8
>
>  /* 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 8bb9594..dacd7cf 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -642,6 +642,8 @@ void __init kvm_spinlock_init(void)
>  {
>         if (!kvm_para_available())
>                 return;
> +       if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED))
> +               return;
>         /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>         if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>                 return;
> --
> 2.7.4
>

You should also add a cpuid flag in kvm part.

Regards,
Wanpeng Li

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