RE: [PATCH 2/5] X86: Hyper-V: Enable IPI enlightenments

[KY Srinivasan]

> -----Original Message-----
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Thursday, April 26, 2018 3:09 PM
> To: KY Srinivasan <kys@xxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> apw@xxxxxxxxxxxxx; jasowang@xxxxxxxxxx; hpa@xxxxxxxxx; Stephen
> Hemminger <sthemmin@xxxxxxxxxxxxx>; Michael Kelley (EOSG)
> <Michael.H.Kelley@xxxxxxxxxxxxx>; vkuznets@xxxxxxxxxx
> Subject: Re: [PATCH 2/5] X86: Hyper-V: Enable IPI enlightenments
> 
> On Wed, 25 Apr 2018, kys@xxxxxxxxxxxxxxxxx wrote:
> > +static int __send_ipi_mask(const struct cpumask *mask, int vector)
> > +{
> > +	int cur_cpu, vcpu;
> > +	struct ipi_arg_non_ex **arg;
> > +	struct ipi_arg_non_ex *ipi_arg;
> > +	int ret = 1;
> 
> So this indicates whether __send_ipi_mask() can send to @mask or not. So
> please make it a bool and let it return false when it does not work, true
> otherwise. If you had used -Exxxx then it would have been more obvious,
> but
> this is really a boolean decision.

Agreed.
> 
> > +	unsigned long flags;
> > +
> > +	if (cpumask_empty(mask))
> > +		return 0;
> > +
> > +	if (!hv_hypercall_pg)
> > +		return ret;
> > +
> > +	if ((vector < HV_IPI_LOW_VECTOR) || (vector >
> HV_IPI_HIGH_VECTOR))
> > +		return ret;
> > +
> > +	local_irq_save(flags);
> > +	arg = (struct ipi_arg_non_ex
> **)this_cpu_ptr(hyperv_pcpu_input_arg);
> > +
> > +	ipi_arg = *arg;
> > +	if (unlikely(!ipi_arg))
> > +		goto ipi_mask_done;
> > +
> > +
> 
> Stray newline
> 
> > +	ipi_arg->vector = vector;
> > +	ipi_arg->reserved = 0;
> > +	ipi_arg->cpu_mask = 0;
> > +
> > +	for_each_cpu(cur_cpu, mask) {
> > +		vcpu = hv_cpu_number_to_vp_number(cur_cpu);
> > +		if (vcpu >= 64)
> > +			goto ipi_mask_done;
> 
> This is completely magic and deserves a comment.
> 
> > +
> > +		__set_bit(vcpu, (unsigned long *)&ipi_arg->cpu_mask);
> > +	}
> > +
> > +	ret = hv_do_hypercall(HVCALL_SEND_IPI, ipi_arg, NULL);
> > +
> > +ipi_mask_done:
> > +	local_irq_restore(flags);
> > +	return ret;
> > +}
> 
> ....
> 
> >  static int hv_cpu_init(unsigned int cpu)
> >  {
> >  	u64 msr_vp_index;
> >  	struct hv_vp_assist_page **hvp =
> &hv_vp_assist_page[smp_processor_id()];
> > +	void **input_arg;
> > +
> > +	input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
> > +	*input_arg = page_address(alloc_page(GFP_ATOMIC));
> 
> This is called from the cpu hotplug thread and there is no need for an
> atomic allocation. Please use GFP_KERNEL.
> 
> >  	hv_get_vp_index(msr_vp_index);
> >
> > @@ -217,6 +224,10 @@ static int hv_cpu_die(unsigned int cpu)
> >  {
> >  	struct hv_reenlightenment_control re_ctrl;
> >  	unsigned int new_cpu;
> > +	void **input_arg;
> > +
> > +	input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
> > +	free_page((unsigned long)*input_arg);
> 
> Hrm. Again this is called from the CPU hotplug thread when the cou is about
> to go down. But you can be scheduled out after free() and before disabling
> the assist thing below and the pointer persist. There is no guarantee that
> nothing sends an IPI anymore after this point.
> 
> So you have two options here:
> 
>   1) Disable interrupts, get the pointer, set the per cpu pointer to NULL,
>      reenable interruots and free the page
I will implement this approach.
> 
>   2) Keep the page around and check for it in the CPU UP path and avoid the
>      allocation when the CPU comes online again.
> 
> >  	if (hv_vp_assist_page && hv_vp_assist_page[cpu])
> >  		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
> > @@ -260,6 +271,12 @@ void __init hyperv_init(void)
> >  	if ((ms_hyperv.features & required_msrs) != required_msrs)
> >  		return;
> >
> > +	/* Allocate the per-CPU state for the hypercall input arg */
> > +	hyperv_pcpu_input_arg = alloc_percpu(void  *);
> > +
> > +	if (hyperv_pcpu_input_arg == NULL)
> > +		return;
> 
> Huch. When that allocation fails, you return and ignore the rest of the
> function which has been there before. Weird decision.
I should have explained this. Failure of this allocation means that we would not have the
per-cpu hypercall input page which in turn would mean that we would not be able to invoke
any hypercalls. 

Regards,

K. Y