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Re: [Xen-devel] [PATCH 0/7] xen/arm: CPU hotplug fixes

On 11/04/18 17:37, Mirela Simonovic wrote:
Hi Julien,


May I ask you to configure your mail client to use > for quoting and use plain text? Otherwise, this is going to be really difficult to follow the discussion after few round (see already below).

On Wed, Apr 11, 2018 at 6:02 PM, Julien Grall <julien.grall@xxxxxxx <mailto:julien.grall@xxxxxxx>> wrote:


    On 11/04/18 16:58, Mirela Simonovic wrote:

        On 04/11/2018 05:07 PM, Julien Grall wrote:

            On 11/04/18 14:19, Mirela Simonovic wrote:

        Migrating interrupts when turning off a CPU already works.
        However, when a CPU is turned back on there is no interrupt
        migration back to the hotplugged CPU - all interrupts will
        remain routed to the CPU#0.
        Patch 7/7 fixes this

    What do you mean by all interrupts? Interrupts routed to guest will
    always follow the vCPU. So are you sure they are going to be
    migrated when that vCPU is paused/off?

Just to make sure we're on the same page - this is about hotplugging physical CPUs. Hotplugging vCPUs using virtual PSCI CPU_OFF interface is already implemented and unrelated to this series.

Yes, we are on the same page :). I was just wondering what happen to interrupt routed to that pCPU.

Assuming that system has 2 pCPUs by 'all interrupts' I mean interrupts that were targeted to the pCPU#0 and pCPU#1 prior to doing any hotplug.

For example, if a guest is pinned to pCPU#1 an interrupt of a device it owns will be targeted to pCPU#1. When pCPU#1 is turned off that interrupt will be migrated to pCPU#0. pCPU#0 finalizes the suspend and receives wake-up interrupts. However, when CPU#1 is turned back on that interrupt will remain targeted to the CPU#0, which I assumed is wrong.
The scenario described here is also how I tested this.

    Can you give the path in Xen doing that?

Sure, here is a backtrace (dumped on the CPU being turned off):
     0  0x2603dc arch_move_irqs(): vgic.c, line 309
     1  0x22ee58 sched_move_irqs()+20: schedule.c, line 303
     2  0x2318e8 cpu_disable_scheduler()+1000: schedule.c, line 586
     3  0x2318e8 cpu_disable_scheduler()+1000: schedule.c, line 586
     4  0x25aff8 __cpu_disable()+96: smpboot.c, line 386
     5  0x201608 take_cpu_down()+52: cpu.c, line 75
     6  0x23426c stopmachine_action()+188: stop_machine.c, line 159
     7  0x235858 do_tasklet_work()+176: tasklet.c, line 94
     8  0x235c80 do_tasklet()+104: tasklet.c, line 126
     9  0x24daec idle_loop()+144: domain.c, line 72
    10  0x25b1f8 start_secondary()+404: smpboot.c, line 368

So this cover interrupt routed to a virtual CPU. However, this does not handle interrupts used by Xen. How do you handle them?

For instance SMMUs IRQ might be routed to other interrupt than CPU #0.


    If the former, it would be nice to get the code you used. If the
    latter, then having a hack patch to test that code would be nice.
    Ideally, you want to plug that in the SYSCTL interface for
    out-of-box testing.

Ok, I have never used that but I'll try to figure it out. I may come up with additional questions.

You might want to have a look at the x86 version XEN_SYSCTL_cpu_hotplug in x86/systcl.c.


Julien Grall

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