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

[Dmitry Safonov]

On 6/14/19 11:08 AM, Vitaly Kuznetsov wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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