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SubjectRe: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall
Hi, Michael,

Thanks a lot for the review and comments! Let us sync with Hyper-V team
to confirm these suspicious points.

BRs,
Sun Yi

On 18-10-24 16:53:00, Michael Kelley wrote:
> From: Yi Sun <yi.y.sun@linux.intel.com> 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.
>
I will send mail to Hyper-V team to clarify these.

> > 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, I should declare it as 'unsigned int'. 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.
>
> > + 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.
>
> > +
> > + 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.
>
> Michael

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