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

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Date:   Mon, 22 Oct 2018 09:32:31 +0200
From:   Juergen Gross <jgross@...e.com>
To:     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>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

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