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feat(hal-x86_64): crosscheck PIT/CPUID LAPIC freq (#499)
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This commit changes the local APIC calibration code to attempt PIT
calibration even when we find CPUID leaves indicating the frequency.
This way, we can cross-check them and see how sloppy the PIT calibration
is. It turns out its pretty bad.
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@hawkw
hawkw committed Dec 31, 2024
1 parent 0363e82 commit d9dac1d
Showing 1 changed file with 91 additions and 43 deletions.
134 changes: 91 additions & 43 deletions hal-x86_64/src/interrupt/apic/local.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -127,11 +127,64 @@ impl LocalApic {
tracing::info!(base = ?self.base, spurious_vector, "local APIC enabled");
}

fn timer_frequency_hz(&self) -> u32 {
/// Calibrate the timer frequency using the PIT.
#[inline(always)]
fn calibrate_frequency_hz_pit(&self) -> u32 {
use register::*;
tracing::debug!("calibrating APIC timer frequency using PIT...");

let cpuid = CpuId::new();
// lock the PIT now, before actually starting the timer IRQ, so that we
// don't include any time spent waiting for the PIT lock.
//
// since we only run this code on startup, before any other cores have
// been started, this probably never actually waits for a lock. but...we
// should do it the right way, anyway.
let mut pit = crate::time::PIT.lock();

unsafe {
// start the timer

// set timer divisor to 16
self.write_register(TIMER_DIVISOR, 0b11);
self.write_register(
LVT_TIMER,
LvtTimer::new()
.with(LvtTimer::MODE, TimerMode::OneShot)
.with(LvtTimer::MASKED, false),
);
// set initial count to -1
self.write_register(TIMER_INITIAL_COUNT, -1i32 as u32);
}

// use the PIT to sleep for 10ms
pit.sleep_blocking(Duration::from_millis(10))
.expect("the PIT should be able to send a 10ms interrupt...");

unsafe {
// stop the timer
self.write_register(
LVT_TIMER,
LvtTimer::new()
.with(LvtTimer::MODE, TimerMode::OneShot)
.with(LvtTimer::MASKED, true),
);
}

let elapsed_ticks = unsafe { self.register(TIMER_CURRENT_COUNT).read() };
// since we slept for ten milliseconds, each tick is 10 kHz. we don't
// need to account for the divisor since we ran the timer at that
// divisor already.
let ticks_per_10ms = (-1i32 as u32).wrapping_sub(elapsed_ticks);
tracing::debug!(?ticks_per_10ms);
// convert the frequency to Hz.
let frequency_hz = ticks_per_10ms * 100;
tracing::debug!(frequency_hz, "calibrated local APIC timer using PIT");
frequency_hz
}

#[inline(always)]
fn calibrate_frequency_hz_cpuid() -> Option<u32> {
let cpuid = CpuId::new();
if let Some(freq_khz) = cpuid.get_hypervisor_info().and_then(|hypervisor| {
tracing::trace!("CPUID contains hypervisor info");
let freq = hypervisor.apic_frequency();
Expand All @@ -143,9 +196,9 @@ impl LocalApic {
let frequency_hz = (freq_khz.get() * 1000) / Self::TIMER_DIVISOR;
tracing::debug!(
frequency_hz,
"determined APIC frequency from CPUID hypervisor info"
"determined APIC timer frequency from CPUID hypervisor info"
);
return frequency_hz;
return Some(frequency_hz);
}

// XXX ELIZA THIS IS TSC FREQUENCY, SO IDK IF THAT'S RIGHT?
Expand All @@ -167,51 +220,46 @@ impl LocalApic {

// CPUID didn't help, so fall back to calibrating the APIC frequency
// using the PIT.
tracing::debug!("calibrating APIC timer frequency using PIT...");

// lock the PIT now, before actually starting the timer IRQ, so that we
// don't include any time spent waiting for the PIT lock.
//
// since we only run this code on startup, before any other cores have
// been started, this probably never actually waits for a lock. but...we
// should do it the right way, anyway.
let mut pit = crate::time::PIT.lock();

unsafe {
// set timer divisor to 16
self.write_register(TIMER_DIVISOR, 0b11);
// set initial count to -1
self.write_register(TIMER_INITIAL_COUNT, -1i32 as u32);
None
}

// start the timer
self.write_register(
LVT_TIMER,
LvtTimer::new().with(LvtTimer::MODE, TimerMode::OneShot),
fn timer_frequency_hz(&self) -> u32 {
// How sloppy do we expect the PIT frequency calibration to be?
// If the delta between the CPUID frequency and the frequency we
// determined using PIT calibration is > this value, we'll yell about
// it.
const PIT_SLOPPINESS: u32 = 1000;

// Start out by calibrating the APIC frequency using the PIT.
let pit_frequency_hz = self.calibrate_frequency_hz_pit();

// See if we can get something from CPUID.
let Some(cpuid_frequency_hz) = Self::calibrate_frequency_hz_cpuid() else {
tracing::info!(
pit.frequency_hz = pit_frequency_hz,
"CPUID does not indicate APIC timer frequency; using PIT \
calibration only"
);
}

// use the PIT to sleep for 10ms
pit.sleep_blocking(Duration::from_millis(10))
.expect("the PIT should be able to send a 10ms interrupt...");
return pit_frequency_hz;
};

unsafe {
// stop the timer
self.write_register(
LVT_TIMER,
LvtTimer::new().with(LvtTimer::MODE, TimerMode::Periodic),
// Cross-check the PIT calibration result and CPUID value.
let distance = if pit_frequency_hz > cpuid_frequency_hz {
pit_frequency_hz - cpuid_frequency_hz
} else {
cpuid_frequency_hz - pit_frequency_hz
};
if distance > PIT_SLOPPINESS {
tracing::warn!(
pit.frequency_hz = pit_frequency_hz,
cpuid.frequency_hz = cpuid_frequency_hz,
distance,
"APIC timer frequency from PIT calibration differs substantially \
from CPUID!"
);
}

let elapsed_ticks = unsafe { self.register(TIMER_CURRENT_COUNT).read() };
// since we slept for ten milliseconds, each tick is 10 kHz. we don't
// need to account for the divisor since we ran the timer at that
// divisor already.
let ticks_per_10ms = (-1i32 as u32).wrapping_sub(elapsed_ticks);
tracing::debug!(?ticks_per_10ms);
// convert the frequency to Hz.
let frequency_hz = ticks_per_10ms * 100;
tracing::debug!(frequency_hz, "calibrated local APIC timer using PIT");
frequency_hz
cpuid_frequency_hz
}

#[tracing::instrument(
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