Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

Dmitry Safonov
2019-06-14T11:51:04Z

On 6/14/19 11:08 AM, Vitaly Kuznetsov wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
>
>> @@ -182,7 +182,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
>> struct hv_reenlightenment_control re_ctrl = {
>> .vector = HYPERV_REENLIGHTENMENT_VECTOR,
>> .enabled = 1,
>> - .target_vp = hv_vp_index[smp_processor_id()]
>> + .target_vp = hv_vp_index[raw_smp_processor_id()]
>> };
>> struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
>>
>
> Yes, this should do, thanks! I'd also suggest to leave a comment like
> /*
> * This function can get preemted and migrate to a different CPU
> * but this doesn't matter. We just need to assign
> * reenlightenment notification to some online CPU. In case this
> * CPU goes offline, hv_cpu_die() will re-assign it to some
> * other online CPU.
> */

What if the cpu goes down just before wrmsrl()?
I mean, hv_cpu_die() will reassign another cpu, but this thread will be
resumed on some other cpu and will write cpu number which is at that
moment already down?

(probably I miss something)

And I presume it's guaranteed that during hv_cpu_die() no other cpu may
go down:
: new_cpu = cpumask_any_but(cpu_online_mask, cpu);
: re_ctrl.target_vp = hv_vp_index[new_cpu];
: wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));

--
Dima