linux-kernel - Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

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Date:   Fri, 19 Oct 2018 16:20:52 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Yi Sun <yi.y.sun@...ux.intel.com>, linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, tglx@...utronix.de, chao.p.peng@...el.com,
        chao.gao@...el.com, isaku.yamahata@...el.com,
        michael.h.kelley@...rosoft.com, tianyu.lan@...rosoft.com,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

On 19/10/2018 15:13, Yi Sun wrote:
> The HvNotifyLongSpinWait hypercall (HVCALL_NOTIFY_LONG_SPIN_WAIT)
> is used by a guest OS to notify the hypervisor that the calling
> virtual processor is attempting to acquire a resource that is
> potentially held by another virtual processor within the same
> Virtual Machine. This scheduling hint improves the scalability of
> VMs with more than one virtual processor on Hyper-V.
> 
> Per MSFT TLFS, the retry number (SpinWaitInfo) is sent to hypervisor
> only when the retry number exceeds the recommended number. If
> recommended number is 0xFFFFFFFF, never retry.
> 
> Signed-off-by: Yi Sun <yi.y.sun@...ux.intel.com>
> Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Michael Kelley (EOSG) <Michael.H.Kelley@...rosoft.com>
> Cc: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/hyperv/hv_spinlock.c       | 18 ++++++++++++++++++
>  arch/x86/include/asm/mshyperv.h     |  1 +
>  kernel/locking/qspinlock_paravirt.h | 10 ++++++++++
>  3 files changed, 29 insertions(+)
> 
> diff --git a/arch/x86/hyperv/hv_spinlock.c b/arch/x86/hyperv/hv_spinlock.c
> index a861b04..723dccb 100644
> --- a/arch/x86/hyperv/hv_spinlock.c
> +++ b/arch/x86/hyperv/hv_spinlock.c
> @@ -18,6 +18,24 @@
>  
>  static bool __initdata hv_pvspin = true;
>  
> +bool hv_notify_long_spin_wait(int retry_num)
> +{
> +	/*
> +	 * Per MSFT TLFS, the SpinWaitInfo is sent to hypervisor only when
> +	 * the retry number exceeds the recommended number.
> +	 *
> +	 * If recommended number is 0xFFFFFFFF, never retry.
> +	 */
> +	if (ms_hyperv.num_spin_retry == HYPERV_SPINLOCK_RETRY_NEVER)
> +		return false;
> +
> +	if ((0 == retry_num % ms_hyperv.num_spin_retry) && retry_num)
> +		hv_do_fast_hypercall8(HVCALL_NOTIFY_LONG_SPIN_WAIT,
> +				      retry_num);
> +
> +	return true;
> +}
> +
>  static void hv_qlock_kick(int cpu)
>  {
>  	apic->send_IPI(cpu, X86_PLATFORM_IPI_VECTOR);
> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
> index f909365..bd87868 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -356,6 +356,7 @@ static inline int cpumask_to_vpset(struct hv_vpset *vpset,
>  void hv_apic_init(void);
>  void __init hv_init_spinlocks(void);
>  bool hv_vcpu_is_preempted(int vcpu);
> +bool hv_notify_long_spin_wait(int retry_num);
>  #else
>  static inline void hv_apic_init(void) {}
>  #endif
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index 0130e48..9e88c7e 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -7,6 +7,8 @@
>  #include <linux/bootmem.h>
>  #include <linux/debug_locks.h>
>  
> +#include <asm/mshyperv.h>
> +
>  /*
>   * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
>   * of spinning them.
> @@ -305,6 +307,10 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
>  				wait_early = true;
>  				break;
>  			}
> +#if defined(CONFIG_X86_64) && defined(CONFIG_PARAVIRT_SPINLOCKS) && IS_ENABLED(CONFIG_HYPERV)
> +			if (!hv_notify_long_spin_wait(SPIN_THRESHOLD - loop))
> +				break;
> +#endif

I don't like that. Why should a KVM or Xen guest call into a hyperv
specific function?

Can't you move this to existing hyperv specific paravirt hooks?


Juergen