From: Mark Kettenis Subject: Re: Improving time keeping in x86 VMs To: Stefan Fritsch Cc: mlarkin@nested.page, dv@sisu.io, tech@openbsd.org, scottcheloha@gmail.com, tedu@tedunangst.com Date: Wed, 21 May 2025 17:12:52 +0200 > Date: Wed, 21 May 2025 17:00:00 +0200 (CEST) > From: Stefan Fritsch > > Hi Mark, > > On Tue, 20 May 2025, Mark Kettenis wrote: > > Thanks, that cleans up things nicely. > > > > > The updated diff below is only the non-vmm(4) part, because the vmm(4) > > > part needs more discussion. I will reply to mlarkin's mail about that. > > > > I think splitting up the diff that way makes a lot of sense. > > > > A few (mostly cosmetic) nits below. > > thanks for your review. Updated diff below. Thanks that looks good to me. ok kettenis@, but you probably also want an ok from someone with more virtualization knowledge. > diff --git a/sys/arch/i386/include/cpufunc.h b/sys/arch/i386/include/cpufunc.h > index 098b5982d20..dca6a8ee473 100644 > --- a/sys/arch/i386/include/cpufunc.h > +++ b/sys/arch/i386/include/cpufunc.h > @@ -229,6 +229,15 @@ rdtsc(void) > return (tsc); > } > > +static inline uint64_t > +rdtsc_lfence(void) > +{ > + uint64_t tsc; > + > + __asm volatile("lfence; rdtsc" : "=A" (tsc)); > + return tsc; > +} > + > static __inline void > wrmsr(u_int msr, u_int64_t newval) > { > diff --git a/sys/dev/pv/pvclock.c b/sys/dev/pv/pvclock.c > index 6d8dd432900..467c641d348 100644 > --- a/sys/dev/pv/pvclock.c > +++ b/sys/dev/pv/pvclock.c > @@ -22,6 +22,7 @@ > > #include > #include > +#include > #include > > #include > @@ -35,13 +36,65 @@ > #define PMAP_NOCRYPT 0 > #endif > > -uint pvclock_lastcount; > +#if defined(__amd64__) > + > +static inline uint64_t > +pvclock_atomic_load(volatile uint64_t *ptr) > +{ > + return *ptr; > +} > + > +static inline uint64_t > +pvclock_atomic_cas(volatile uint64_t *p, uint64_t e, > + uint64_t n) > +{ > + return atomic_cas_ulong((volatile unsigned long *)p, e, n); > +} > + > +#elif defined(__i386__) > + > +/* > + * We are running on virtualization. Therefore we can assume that we > + * have cmpxchg8b, available on pentium and newer. > + */ > +static inline uint64_t > +pvclock_atomic_load(volatile uint64_t *ptr) > +{ > + uint64_t val; > + __asm__ volatile ("movl %%ebx,%%eax; movl %%ecx, %%edx; " > + "lock cmpxchg8b %1" : "=&A" (val) : "m" (*ptr)); > + return val; > +} > + > +static inline uint64_t > +pvclock_atomic_cas(volatile uint64_t *p, uint64_t e, > + uint64_t n) > +{ > + __asm volatile("lock cmpxchg8b %1" : "+A" (e), "+m" (*p) > + : "b" ((uint32_t)n), "c" ((uint32_t)(n >> 32))); > + return (e); > +} > + > +#else > +#error "pvclock: unsupported x86 architecture?" > +#endif > + > + > +uint64_t pvclock_lastcount; > + > +struct pvpage { > + struct pvclock_time_info ti; > + struct pvclock_wall_clock wc; > +}; > > struct pvclock_softc { > struct device sc_dev; > - void *sc_time; > + struct pvpage *sc_page; > paddr_t sc_paddr; > struct timecounter *sc_tc; > + struct ksensordev sc_sensordev; > + struct ksensor sc_sensor; > + struct timeout sc_tick; > }; > > #define DEVNAME(_s) ((_s)->sc_dev.dv_xname) > @@ -50,12 +103,16 @@ int pvclock_match(struct device *, void *, void *); > void pvclock_attach(struct device *, struct device *, void *); > int pvclock_activate(struct device *, int); > > +uint64_t pvclock_get(struct timecounter *); > uint pvclock_get_timecount(struct timecounter *); > +void pvclock_tick_hook(struct device *); > > static inline uint32_t > pvclock_read_begin(const struct pvclock_time_info *); > static inline int > pvclock_read_done(const struct pvclock_time_info *, uint32_t); > +static inline uint64_t > + pvclock_scale_delta(uint64_t, uint32_t, int); > > const struct cfattach pvclock_ca = { > sizeof(struct pvclock_softc), > @@ -123,28 +180,31 @@ void > pvclock_attach(struct device *parent, struct device *self, void *aux) > { > struct pvclock_softc *sc = (struct pvclock_softc *)self; > + struct pv_attach_args *pva = aux; > struct pvclock_time_info *ti; > - paddr_t pa; > + paddr_t pa; > uint32_t version; > uint8_t flags; > struct vm_page *page; > + struct pvbus_hv *kvm; > > page = uvm_pagealloc(NULL, 0, NULL, 0); > if (page == NULL) > goto err; > - sc->sc_time = km_alloc(PAGE_SIZE, &kv_any, &kp_none, &kd_nowait); > - if (sc->sc_time == NULL) > + sc->sc_page = km_alloc(PAGE_SIZE, &kv_any, &kp_none, &kd_nowait); > + if (sc->sc_page == NULL) > goto err; > > pa = VM_PAGE_TO_PHYS(page); > - pmap_kenter_pa((vaddr_t)sc->sc_time, pa | PMAP_NOCRYPT, > + pmap_kenter_pa((vaddr_t)sc->sc_page, pa | PMAP_NOCRYPT, > PROT_READ | PROT_WRITE); > - memset(sc->sc_time, 0, PAGE_SIZE); > + pmap_update(pmap_kernel()); > + memset(sc->sc_page, 0, PAGE_SIZE); > > wrmsr(KVM_MSR_SYSTEM_TIME, pa | PVCLOCK_SYSTEM_TIME_ENABLE); > sc->sc_paddr = pa; > > - ti = sc->sc_time; > + ti = &sc->sc_page->ti; > do { > version = pvclock_read_begin(ti); > flags = ti->ti_flags; > @@ -168,6 +228,22 @@ pvclock_attach(struct device *parent, struct device *self, void *aux) > > tc_init(sc->sc_tc); > > + /* > + * The openbsd vmm pvclock does not support the WALL_CLOCK msr, > + * therefore we look only for kvm. > + */ > + kvm = &pva->pva_hv[PVBUS_KVM]; > + if (kvm->hv_features & (1 << KVM_FEATURE_CLOCKSOURCE2)) { > + strlcpy(sc->sc_sensordev.xname, sc->sc_dev.dv_xname, > + sizeof(sc->sc_sensordev.xname)); > + sc->sc_sensor.type = SENSOR_TIMEDELTA; > + sc->sc_sensor.status = SENSOR_S_UNKNOWN; > + sensor_attach(&sc->sc_sensordev, &sc->sc_sensor); > + sensordev_install(&sc->sc_sensordev); > + > + config_mountroot(self, pvclock_tick_hook); > + } > + > printf("\n"); > return; > err: > @@ -211,8 +287,36 @@ pvclock_read_done(const struct pvclock_time_info *ti, > return (ti->ti_version == version); > } > > -uint > -pvclock_get_timecount(struct timecounter *tc) > +static inline uint64_t > +pvclock_scale_delta(uint64_t delta, uint32_t mul_frac, int shift) > +{ > + uint64_t lower, upper; > + > + if (shift < 0) > + delta >>= -shift; > + else > + delta <<= shift; > + > + lower = ((uint64_t)mul_frac * ((uint32_t)delta)) >> 32; > + upper = (uint64_t)mul_frac * (delta >> 32); > + return lower + upper; > +} > + > +static uint64_t > +pvclock_cmp_last(uint64_t ctr) > +{ > + uint64_t last; > + > + do { > + last = pvclock_atomic_load(&pvclock_lastcount); > + if (ctr < last) > + return (last); > + } while (pvclock_atomic_cas(&pvclock_lastcount, last, ctr) != last); > + return (ctr); > +} > + > +uint64_t > +pvclock_get(struct timecounter *tc) > { > struct pvclock_softc *sc = tc->tc_priv; > struct pvclock_time_info *ti; > @@ -220,8 +324,10 @@ pvclock_get_timecount(struct timecounter *tc) > uint32_t version, mul_frac; > int8_t shift; > uint8_t flags; > + int s; > > - ti = sc->sc_time; > + ti = &sc->sc_page->ti; > + s = splhigh(); > do { > version = pvclock_read_begin(ti); > system_time = ti->ti_system_time; > @@ -229,26 +335,63 @@ pvclock_get_timecount(struct timecounter *tc) > mul_frac = ti->ti_tsc_to_system_mul; > shift = ti->ti_tsc_shift; > flags = ti->ti_flags; > + delta = rdtsc_lfence(); > } while (!pvclock_read_done(ti, version)); > + splx(s); > > /* > * The algorithm is described in > - * linux/Documentation/virtual/kvm/msr.txt > + * linux/Documentation/virt/kvm/x86/msr.rst > */ > - delta = rdtsc() - tsc_timestamp; > - if (shift < 0) > - delta >>= -shift; > + if (delta > tsc_timestamp) > + delta -= tsc_timestamp; > else > - delta <<= shift; > - ctr = ((delta * mul_frac) >> 32) + system_time; > + delta = 0; > + ctr = pvclock_scale_delta(delta, mul_frac, shift) + system_time; > > if ((flags & PVCLOCK_FLAG_TSC_STABLE) != 0) > return (ctr); > > - if (ctr < pvclock_lastcount) > - return (pvclock_lastcount); > + return pvclock_cmp_last(ctr); > +} > > - atomic_swap_uint(&pvclock_lastcount, ctr); > +uint > +pvclock_get_timecount(struct timecounter *tc) > +{ > + return (pvclock_get(tc)); > +} > > - return (ctr); > +void > +pvclock_tick(void *arg) > +{ > + struct pvclock_softc *sc = arg; > + struct timespec ts; > + struct pvclock_wall_clock *wc = &sc->sc_page->wc; > + int64_t value; > + > + wrmsr(KVM_MSR_WALL_CLOCK, sc->sc_paddr + offsetof(struct pvpage, wc)); > + while (wc->wc_version & 0x1) > + virtio_membar_sync(); > + if (wc->wc_sec) { > + nanotime(&ts); > + value = TIMESPEC_TO_NSEC(&ts) - > + SEC_TO_NSEC(wc->wc_sec) - wc->wc_nsec - > + pvclock_get(&pvclock_timecounter); > + > + TIMESPEC_TO_TIMEVAL(&sc->sc_sensor.tv, &ts); > + sc->sc_sensor.value = value; > + sc->sc_sensor.status = SENSOR_S_OK; > + } else > + sc->sc_sensor.status = SENSOR_S_UNKNOWN; > + > + timeout_add_sec(&sc->sc_tick, 15); > +} > + > +void > +pvclock_tick_hook(struct device *self) > +{ > + struct pvclock_softc *sc = (struct pvclock_softc *)self; > + > + timeout_set(&sc->sc_tick, pvclock_tick, sc); > + pvclock_tick(sc); > } >