linux-kernel - Re: [PATCH 3/4] paravirt: add virt_spin

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Message-ID: <3eee978b-e570-5675-7840-c448375b02fc@redhat.com>
Date:   Tue, 5 Sep 2017 10:02:57 -0400
From:   Waiman Long <longman@...hat.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        virtualization@...ts.linux-foundation.org
Cc:     jeremy@...p.org, chrisw@...s-sol.org, akataria@...are.com,
        rusty@...tcorp.com.au, boris.ostrovsky@...cle.com, hpa@...or.com,
        tglx@...utronix.de, mingo@...hat.com, peterz@...radead.org
Subject: Re: [PATCH 3/4] paravirt: add virt_spin_lock pvops function

On 09/05/2017 09:24 AM, Juergen Gross wrote:
> There are cases where a guest tries to switch spinlocks to bare metal
> behavior (e.g. by setting "xen_nopvspin" boot parameter). Today this
> has the downside of falling back to unfair test and set scheme for
> qspinlocks due to virt_spin_lock() detecting the virtualized
> environment.
>
> Make virt_spin_lock() a paravirt operation in order to enable users
> to select an explicit behavior like bare metal.
>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/include/asm/paravirt.h       |  5 ++++
>  arch/x86/include/asm/paravirt_types.h |  1 +
>  arch/x86/include/asm/qspinlock.h      | 48 ++++++++++++++++++++++++-----------
>  arch/x86/kernel/paravirt-spinlocks.c  | 14 ++++++++++
>  arch/x86/kernel/smpboot.c             |  2 ++
>  5 files changed, 55 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index c25dd22f7c70..d9e954fb37df 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -725,6 +725,11 @@ static __always_inline bool pv_vcpu_is_preempted(long cpu)
>  	return PVOP_CALLEE1(bool, pv_lock_ops.vcpu_is_preempted, cpu);
>  }
>  
> +static __always_inline bool pv_virt_spin_lock(struct qspinlock *lock)
> +{
> +	return PVOP_CALLEE1(bool, pv_lock_ops.virt_spin_lock, lock);
> +}
> +
>  #endif /* SMP && PARAVIRT_SPINLOCKS */
>  
>  #ifdef CONFIG_X86_32
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index 19efefc0e27e..928f5e7953a7 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -319,6 +319,7 @@ struct pv_lock_ops {
>  	void (*kick)(int cpu);
>  
>  	struct paravirt_callee_save vcpu_is_preempted;
> +	struct paravirt_callee_save virt_spin_lock;
>  } __no_randomize_layout;
>  
>  /* This contains all the paravirt structures: we get a convenient
> diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
> index 48a706f641f2..fbd98896385c 100644
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -17,6 +17,25 @@ static inline void native_queued_spin_unlock(struct qspinlock *lock)
>  	smp_store_release((u8 *)lock, 0);
>  }
>  
> +static inline bool native_virt_spin_lock(struct qspinlock *lock)
> +{
> +	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
> +		return false;
> +

I think you can take the above if statement out as you has done test in
native_pv_lock_init(). So the test will also be false here.

As this patch series is x86 specific, you should probably add "x86/" in
front of paravirt in the patch titles.

Cheers,
Longman