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

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Date:   Mon, 22 Oct 2018 12:31:45 -0400
From:   Waiman Long <longman@...hat.com>
To:     Juergen Gross <jgross@...e.com>, Yi Sun <yi.y.sun@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, 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>,
        Peter Zijlstra <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

On 10/22/2018 03:32 AM, Juergen Gross wrote:
> On 22/10/2018 03:53, Yi Sun wrote:
>> On 18-10-19 16:20:52, Juergen Gross wrote:
>>> On 19/10/2018 15:13, Yi Sun wrote:
>> [...]
>>
>>>> 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?
>>>
>> hv_notify_long_spin_wait() must be called in this loop but it seems
>> there is no appropriate existing paravirt hook here. So, can I add
>> one more hook in pv_lock_ops to do this notification?
> vcpu_is_preempted() is already part of this loop. And this is a paravirt
> hook. Can't you make use of that? This might require adding another
> parameter to this hook, but I'd prefer that over another pv-spinlock
> hook.
>
> Adding some more locking maintainers and Waiman to the Cc: list.
>
>
> Juergen
>
I agree with Juergen on that. I would suggest rename the
vcpu_is_preempted hook into a more generic vcpu_stop_spinning, perhaps,
so different hypervisors can act on the information accordingly. Adding
an extra parameter is fine.

Cheers,
Longman