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From: KY Srinivasan <kys@microsoft.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "olaf@aepfle.de" <olaf@aepfle.de>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"apw@canonical.com" <apw@canonical.com>,
	"devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
	"vkuznets@redhat.com" <vkuznets@redhat.com>
Subject: RE: [PATCH 2/5] X86: Hyper-V: Enable IPI enlightenments
Date: Fri, 27 Apr 2018 06:11:17 +0000	[thread overview]
Message-ID: <MW2PR2101MB11131D0D8C0B985B68308C25A08D0@MW2PR2101MB1113.namprd21.prod.outlook.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1804262350040.1596@nanos.tec.linutronix.de>



> -----Original Message-----
> From: Thomas Gleixner <tglx@linutronix.de>
> Sent: Thursday, April 26, 2018 3:09 PM
> To: KY Srinivasan <kys@microsoft.com>
> Cc: x86@kernel.org; gregkh@linuxfoundation.org; linux-
> kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.de;
> apw@canonical.com; jasowang@redhat.com; hpa@zytor.com; Stephen
> Hemminger <sthemmin@microsoft.com>; Michael Kelley (EOSG)
> <Michael.H.Kelley@microsoft.com>; vkuznets@redhat.com
> Subject: Re: [PATCH 2/5] X86: Hyper-V: Enable IPI enlightenments
> 
> On Wed, 25 Apr 2018, kys@linuxonhyperv.com 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

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  reply	other threads:[~2018-04-27  6:11 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-25 18:11 [PATCH 0/5] X86: Hyper-V: APIC enlightenments kys
2018-04-25 18:12 ` [PATCH 1/5] X86: Hyper-V: Enlighten APIC access kys
2018-04-25 18:12   ` [PATCH 2/5] X86: Hyper-V: Enable IPI enlightenments kys
2018-04-26 21:31     ` Dan Carpenter
2018-04-27  6:34       ` KY Srinivasan
2018-04-26 22:08     ` Thomas Gleixner
2018-04-27  6:11       ` KY Srinivasan [this message]
2018-04-26 22:54     ` Michael Kelley (EOSG)
2018-04-27  6:31       ` KY Srinivasan
2018-04-27  6:24     ` kbuild test robot
2018-04-27 11:21     ` kbuild test robot
2018-04-27 11:55     ` kbuild test robot
2018-04-25 18:12   ` [PATCH 3/5] X86: Hyper-V: Enhanced IPI enlightenment kys
2018-04-26 22:16     ` Thomas Gleixner
2018-04-27  6:24       ` KY Srinivasan
2018-04-26 23:25     ` Michael Kelley (EOSG)
2018-04-25 18:12   ` [PATCH 4/5] X86: Hyper-V: Consolidate code for converting cpumask to vpset kys
2018-04-26 22:21     ` Thomas Gleixner
2018-04-25 18:12   ` [PATCH 5/5] X86: Hyper-V: Consolidate the allocation of the hypercall input page kys
2018-04-26 22:23     ` Thomas Gleixner
2018-04-27  6:29       ` KY Srinivasan
2018-04-27 10:58         ` Thomas Gleixner
2018-04-27 11:50     ` kbuild test robot
2018-04-26 21:49   ` [PATCH 1/5] X86: Hyper-V: Enlighten APIC access Thomas Gleixner
2018-04-27  5:52     ` KY Srinivasan
2018-04-26 22:15   ` Michael Kelley (EOSG)
2018-04-26 22:55     ` Thomas Gleixner
2018-04-27  5:44   ` kbuild test robot

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