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Re: [PATCH v4 1/4] x86/APIC: calibrate against platform timer when possible
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 4 Apr 2022 13:55:56 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Mon, 04 Apr 2022 11:56:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 04.04.2022 13:46, Roger Pau Monné wrote:
> On Thu, Mar 31, 2022 at 11:29:44AM +0200, Jan Beulich wrote:
>> +uint64_t __init calibrate_apic_timer(void)
>> +{
>> + uint32_t start, end;
>> + uint64_t count = read_pt_and_tmcct(&start), elapsed;
>> + uint64_t target = CALIBRATE_VALUE(plt_src.frequency), actual;
>> + uint64_t mask = (uint64_t)~0 >> (64 - plt_src.counter_bits);
>> +
>> + /*
>> + * PIT cannot be used here as it requires the timer interrupt to
>> maintain
>> + * its 32-bit software counter, yet here we run with IRQs disabled.
>> + */
>> + if ( using_pit )
>> + return 0;
>> +
>> + while ( ((plt_src.read_counter() - count) & mask) < target )
>> + continue;
>> +
>> + actual = read_pt_and_tmcct(&end) - count;
>
> Don't you need to apply the pt mask here?
Oh, yes, of course. I guess I did clone an earlier mistake from
calibrate_tsc().
>> + elapsed = start - end;
>> +
>> + if ( likely(actual > target) )
>> + {
>> + /* See the comment in calibrate_tsc(). */
>
> I would maybe add that the counters used here might have > 32 bits,
> and hence we need to trim the values for muldiv64 to scale properly.
Sure - I've added "But first scale down values to actually fit
muldiv64()'s input range."
Jan
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