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Re: xen 4.14.3 incorrect (~3x) cpu frequency reported
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 12 Jan 2022 17:55:39 +0100
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- Cc: James Dingwall <james-xen@xxxxxxxxxxxxxx>, alexander.rossa@xxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 12 Jan 2022 16:56:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 10.01.2022 13:37, Roger Pau Monné wrote:
> On Fri, Jan 07, 2022 at 12:39:04PM +0100, Jan Beulich wrote:
>> @@ -415,16 +416,35 @@ static int64_t __init init_hpet(struct p
>>
>> pts->frequency = hpet_rate;
>>
>> +for(i = 0; i < 16; ++i) {//temp
>> count = hpet_read32(HPET_COUNTER);
>> start = rdtsc_ordered();
>> target = count + CALIBRATE_VALUE(hpet_rate);
>> if ( target < count )
>> while ( hpet_read32(HPET_COUNTER) >= count )
>> continue;
>> - while ( hpet_read32(HPET_COUNTER) < target )
>> + while ( (count = hpet_read32(HPET_COUNTER)) < target )
>> continue;
>>
>> - return (rdtsc_ordered() - start) * CALIBRATE_FRAC;
>> + expired = rdtsc_ordered() - start;
>
> There's also a window between the HPET read and the TSC read where an
> SMI/NMI could cause a wrong frequency detection.
It occurred to me only while preparing a draft change that this is an
issue not only for the tail pair of reads, but also for the initial pair.
Hence I'm now actually making a helper function to be used in both
places. This slightly sub-optimal as it means that it's not as easy to
make use of the last HPET read in the loop above, but I guess we can
tolerate that one extra read in preference over more complicated logic.
Jan
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