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

Dmitry Safonov
2019-06-14T14:28:42Z

On 6/14/19 1:27 PM, Peter Zijlstra wrote:
> On Fri, Jun 14, 2019 at 12:50:51PM +0100, Dmitry Safonov wrote:
>> 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));
>
> Then cpus_read_lock() is the right interface, not preempt_disable().
>
> I know you probably can't change the HV interface, but I'm thinking its
> rather daft you have to specify a CPU at all for this. The HV can just
> pick one and send the notification there, who cares.

Heh, I thought cpus_read_lock() is more "internal" api and
preempt_diable() is prefered ;-)

Will send v2 with the suggested comment and cpus_read_lock().

--
Dima