Re: [Xen-devel] [PATCH v2 2/3] x86/vlapic: Keep timer running when switching between one-shot and periodic mode

["Jan Beulich" <jbeulich@xxxxxxxx>]

>> Anthony PERARD <anthony.perard@xxxxxxxxxx> 08/04/17 1:38 PM >>>
>On Thu, Aug 03, 2017 at 09:21:57AM -0600, Jan Beulich wrote:
>> >>> Anthony PERARD <anthony.perard@xxxxxxxxxx> 07/18/17 7:12 PM >>>
>> >@@ -818,6 +840,7 @@ static void vlapic_reg_write(struct vcpu *v,
>> >if ( !vlapic_lvtt_oneshot(vlapic) && !vlapic_lvtt_period(vlapic) )
>> >break;
>>  >
>> >+        vlapic->timer_last_update = hvm_get_guest_time(current);
>> >vlapic_set_reg(vlapic, APIC_TMICT, val);
>>  >
>> >vlapic_update_timer(vlapic, vlapic_get_reg(vlapic, APIC_LVTT));
>> 
>> Why is this addition needed? vlapic_update_timer() sets timer_last_update
>> anyway. As it looks all you want is the value to be non-zero, which can be
>> done with less overhead and should be stated so in a comment.
>
>This is not true, the value is used before been set. It is used to
>calculate how much time have passed since tmict was set. Before been set
>again, there is this:
>time_passed = hvm_get_guest_time(current) - vlapic->timer_last_update;

Hmm, then I'm even more puzzled - the two hvm_get_guest_time() calls
will then result in a small but non-zero delta. Is that really intended?

Jan


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