"Michael Kelley (EOSG)" <Michael.H.Kelley@xxxxxxxxxxxxx> writes:
> From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Tuesday, July 31, 2018 4:20 AM
>>
>> Alternatively, we can get rid of synic_initialized flag altogether:
>> hv_synic_init() never fails in the first place but we can always
>> implement something like:
>>
>> int hv_synic_is_initialized(void) {
>> union hv_synic_scontrol sctrl;
>>
>> hv_get_synic_state(sctrl.as_uint64);
>>
>> return sctrl.enable;
>> }
>>
>> as it doesn't seem that we need to check synic state on _other_ CPUs.
>>
>
> I was trying to decide if there are any arguments in favor of one
> approach vs. the other: a per-cpu flag in memory or checking
> the synic_control "enable" bit. Seems like a wash to me, in which
> case I have a slight preference for the per-cpu flag in memory vs.
> creating another function to return sctrl.enable. But I'm completely
> open to reasons why checking sctrl.enable is better.
Just a few thoughts: reading MSR is definitely slower but we avoid
'shadowing' the state, the reading is always correct. In case there's a
chance the SynIC will get disabled from host side we can only find this
out by doing MSR read. This is a purely theoretical possibility, I
believe, we can go ahead with this patch.
--
Vitaly
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