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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>
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        "marcelo.cerri\@canonical.com" <marcelo.cerri@canonical.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>
Subject: Re: [PATCH char-misc 1/1] Drivers: hv: vmbus: Make synic_initialized flag per-cpu
References: <1533004484-3937-1-git-send-email-mikelley@microsoft.com>
        <87ftzzodnv.fsf@vitty.brq.redhat.com>
        <CY4PR21MB0773EFCF6EA21CF8F9CE01E0DC2D0@CY4PR21MB0773.namprd21.prod.outlook.com>
Date:   Wed, 01 Aug 2018 11:26:13 +0200
In-Reply-To: <CY4PR21MB0773EFCF6EA21CF8F9CE01E0DC2D0@CY4PR21MB0773.namprd21.prod.outlook.com>
        (Michael Kelley's message of "Wed, 1 Aug 2018 05:47:08 +0000")
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"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com> 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