lkml.org 
[lkml]   [2018]   [Apr]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH 2/5] X86: Hyper-V: Enable IPI enlightenments
Date


> -----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

\
 
 \ /
  Last update: 2018-04-27 08:11    [W:0.067 / U:0.836 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site