From: Waldemar Kozaczuk <[email protected]>
Committer: Waldemar Kozaczuk <[email protected]>
Branch: master
hpet: handle wrap-around with 32-bit counter
This patch enhances the hpet clock with 32-bit main
counter to handle wrap-arounds. It does it by maintaining
separate upper 32-bit counter per-cpu and incrementing it
when wrap around is detected.
Signed-off-by: Waldemar Kozaczuk <[email protected]>
---
diff --git a/drivers/hpet.cc b/drivers/hpet.cc
--- a/drivers/hpet.cc
+++ b/drivers/hpet.cc
@@ -18,6 +18,7 @@ extern "C" {
#include <osv/xen.hh>
#include <osv/irqlock.hh>
#include "rtc.hh"
+#include <osv/percpu.hh>
using boost::intrusive::get_parent_from_member;
@@ -33,23 +34,53 @@ class hpetclock : public clock {
#define HPET_COUNTER 0x0f0
-//FIXME: Enhance this class to handle wrap-around
+// The hpet clocks are tricky to implement right. Ideally hpet clocks,
+// especially those with 32-bit main counter, should be avoided in
+// virtualized environments. Unfortunately some hypervisors like
+// hyperkit and other bhyve derivatives provide 32-bit hpet clock only and
+// we would like to support those hypervisors as well to let potential
+// users test OSv on OSX and FreeBSD.
+// The implementation below is a compromise that delivers monotonic
+// reads but may suffer from inaccurate time reads especially when
+// OSv guest gets suspended by host. In essence it maintains it own
+// upper 32-bit counter per cpu which gets incremented every time
+// it gets less than previously saved main counter read from the host.
+// In future we can improve it by handling wrap-around interrupts but
+// but it is not clear if it would be better and especially worth the
effort.
class hpet_32bit_clock : public hpetclock {
public:
- hpet_32bit_clock(mmioaddr_t hpet_mmio_address) :
hpetclock(hpet_mmio_address) {
- debug_early_u64("WARNING: hpet with 32-bit counter will wrap
around in seconds: ",
- (_period * (1UL << 32)) / 1000000000UL);
- }
+ hpet_32bit_clock(mmioaddr_t hpet_mmio_address) :
hpetclock(hpet_mmio_address) {}
+
protected:
virtual s64 time() override __attribute__((no_instrument_function)) {
- return _wall + mmio_getl(_addr + HPET_COUNTER) * _period;
+ return _wall + fetch_counter() * _period;
}
virtual s64 uptime() override __attribute__((no_instrument_function)) {
- return mmio_getl(_addr + HPET_COUNTER) * _period;
+ return fetch_counter() * _period;
}
+private:
+ s64 fetch_counter() {
+ auto last_valid_counter =
(*_last_read_counter).load(std::memory_order_relaxed);
+ auto upper_counter =
(*_upper_32bit_counter).load(std::memory_order_relaxed);
+
+ s64 counter = mmio_getl(_addr + HPET_COUNTER);
+ if (counter < last_valid_counter) {
+ // Wrap-around - increment upper counter
+ (*_upper_32bit_counter).compare_exchange_strong(upper_counter,
upper_counter + 1);
+ }
+
+ (*_last_read_counter).store(counter, std::memory_order_relaxed);
+ return ((*_upper_32bit_counter).load(std::memory_order_relaxed) <<
32) + counter;
+ }
+
+ static percpu<std::atomic<s64>> _upper_32bit_counter;
+ static percpu<std::atomic<s64>> _last_read_counter;
};
+PERCPU(std::atomic<s64>, hpet_32bit_clock::_upper_32bit_counter);
+PERCPU(std::atomic<s64>, hpet_32bit_clock::_last_read_counter);
+
class hpet_64bit_clock : public hpetclock {
public:
hpet_64bit_clock(mmioaddr_t hpet_mmio_address) :
hpetclock(hpet_mmio_address) {}
--
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