git: a8b89dff6ac0 - main - Disable acpi_timer_test by default

[Colin Percival]

On 9/8/21 9:01 AM, John Baldwin wrote:
> On 9/7/21 6:32 PM, Colin Percival wrote:
>>      Disable acpi_timer_test by default
>>           This disables testing the ACPI timer by default, forcing the use of
>>      ACPI-fast rather than ACPI-safe.  The broken-ACPI-timers workaround
>>      can be re-enabled by setting the hw.acpi.timer_test_enabled=1 tunable.
>>           This speeds up the FreeBSD boot process by 140 ms on an EC2 c5.xlarge
>>      instance.
>>           This change will not be MFCed.
>>           Assuming no problems are reported, acpi_timer_test, the associated
>>      tunable, and the ACPI-safe timecounter should be removed in FreeBSD 15.
>>           Relnotes:       The ACPI-safe timer is disabled in favour of
>> ACPI-fast;
>>                      if timekeeping issues are observed, please test with
>>                      hw.acpi.timer_test_enabled=1 in loader.conf and report
>>                      if that fixes the problem.
> 
> Perhaps it should default to '1' for i386 and '0' otherwise?  The relevant
> chipsets were 32-bit only, so this would be a simple way to skip the test for
> modern hardware, and you could probably MFC that safely.

That option was discussed, but I decided that it was probably safer to
keep it enabled by default in 13 in case the test was detecting systems
which are broken in other ways.

Googling for "ACPI-safe" (which shows up if the test fails) finds forum
discussions from the mid-2010s, but it's not clear whether that's due to
very old hardware, new ACPI timer issues, or other timekeeping problems
-- I figured it was best to play it safe for something which would be
going into a stable branch though.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid