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

[Yi Sun]

After syncing with Hyper-V team, we have got answers as below.

On 18-10-24 16:53:00, Michael Kelley wrote:
> From: Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>  Sent: Friday, October 19, 2018 6:14 AM
> > 
> > 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.
> 
> The HvNotifyLongSpinWait hypercall should be understood to be
> advisory only.  As you noted, it is a scheduling hint to the
> hypervisor that some virtual CPU in the VM holds a spin lock.  Even
> though Linux knows which vCPU holds the spin lock, the hypercall
> does not provide a way to give that information to Hyper-V.  The
> hypercall always returns immediately.
> 
> The "retry number" is a bit mis-named in the Hyper-V Top Level
> Functional Spec (TLFS).  It is essentially a threshold value.  Hyper-V is
> saying "don't bother to advise me about the spin lock until you have
> done a certain number of spins."  This threshold prevents
> over-notifying Hyper-V such that the notification becomes somewhat
> meaningless.   It's not immediately clear to me why the hypercall passes
> that value as an input, but maybe it lets the Hyper-V scheduler prioritize
> among vCPUs based on how many times they have spun for a lock.  I
> think we were told that current Hyper-V implementations ignore this
> input value anyway.
> 
> I believe the description of the sentinel value 0xFFFFFFFF in the
> Hyper-V TLFS is incorrect.  Because it is the max possible threshold
> value, that value in the EBX register just means to not ever bother to
> notify.   The description should be "0xFFFFFFFF indicates never to notify."
> The value does *not* indicate anything about retrying to obtain the
> spin lock.
> 
You are right. 0xFFFFFFFF only indicates never to notify. We should not
break the spin loop.

> >  static bool __initdata hv_pvspin = true;
> > 
> > +bool hv_notify_long_spin_wait(int retry_num)
> 
> retry_num should probably be declared as unsigned int.  You
> don't want it to be treated as a negative number if the high
> order bit is set.
> 
Yes, thanks!

> > +{
> > +	/*
> > +	 * 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)
> 
> I don't know if the "%" function is right here.  Your implementation will
> notify Hyper-V on every multiple of num_spin_retry.   The alternative is to
> notify once when the threshold is exceeded, and never again for this
> particular attempt to obtain a spin lock.  We should check with the Hyper-V
> team for which approach they expect to be used.
> 
We should send the notification on every multiple of the recommended
number.

> > +		hv_do_fast_hypercall8(HVCALL_NOTIFY_LONG_SPIN_WAIT,
> > +				      retry_num);
> 
> The Hyper-V TLFS seems to be inconsistent on whether the input parameter
> is 32-bits or 64-bits.   In one place it is typed as UINT64, but in another place
> it is shown as only 4 bytes.  Need to clear this up with the Hyper-V team as
> well.
> 
It is 32-bits.

> > +
> > +	return true;
> 
> I don't see a need for this function to return true vs. false.  Any calling code
> should not change its behavior based on num_spin_retry.   This function will
> either notify Hyper-V or not notify Hyper-V, depending on whether the number
> of attempts to obtain the spinlock meets the threshold.  But calling code will
> do the same thing regardless of whether such a notification is made. 
> 
You are right. I will change it to 'void'.

> Michael