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

Hi Julien,

On Wed, Apr 11, 2018 at 6:02 PM, Julien Grall <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.

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


Calls to enable/disable_nonboot_cpus() functions currently don't exist in
Xen ARM code. This will be added with the suspend to RAM support for ARM.

What would be the way to test that series?

I tested/developed the series on Xilinx Zynq UltraScale+ MPSoC (ZCU102 board), but how would others test that - I don't know. We will have to ask Xilinx for detailed answer to this question.

I am not sure to understand your answer. What I asked, is how did you did turned off the CPU? Did you hack Xen or do you rely on another series?

I rely on the suspend to RAM series that will be submitted after this one is done. Not complete suspend to RAM support is needed in order to do the testing.
Essentially, I have triggered suspend to RAM from Dom0 and the minimum in Xen is to have the suspend support for secondary CPUs (calls to disable/enable_nonboot_cpus).
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.



Julien Grall

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