Waiting loops like the one in flush_command_buffer() will degenerate to infinite ones when used early enough for NOW() to still return constant zero. Make sure the returned value at least monotonically increases. Do this only in get_s_time(), as producing a sane value in get_s_time_fixed() for non-zero inputs won't be reasonably possible. Reported-by: Roger Pau Monné <roger.pau@citrix.com> Signed-off-by: Jan Beulich <jbeulich@suse.com> --- RFC: This breaks at least the TSM_BOOT case printk_start_of_line(), which checks for NOW() returning 0 (falling back to TSM_RAW in this case). For now I have no idea how to avoid this, except that when CPUID leaf 0x15 is available we could leverage that to put in place at least an approximate scale value. Doing so could, however, lead to a discontinuity (returned value moving backwards) once the final scale value was put in place. (Note, however, that such a discontinuity can also result from init_percpu_time() using the BSP's scale value as initial estimate for APs. Then again local_time_calibration() at least makes an attempt at avoiding such.) RFC: While generally the mentioned waiting loops will take longer to time out, on a very fast CPU tight loops may time out too early. RFC: In get_s_time_fixed(), should we perhaps assert that the scale was set? I don't think Fixes: tags should be put here. If we did, we'd have to enumerate all introductions of early uses of NOW() (or get_s_time()), with the exception of those dealing with getting back 0 (which I expect is only printk_start_of_line()). --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -XXX,XX +XXX,XX @@ s_time_t get_s_time_fixed(u64 at_tsc) s_time_t get_s_time(void) { + /* + * Before the TSC scale is set, avoid returning constant 0 (or whatever + * this_cpu(cpu_time).stamp.local_stime is set to). While the returned + * value is in no way representing time, it at least increases + * monotonically, thus avoiding e.g. waiting loops to degenerate to + * entirely infinite ones. + */ + if ( unlikely(!this_cpu(cpu_time).tsc_scale.mul_frac) ) + { + static s_time_t counter; + + return arch_fetch_and_add(&counter, 1); + } + return get_s_time_fixed(0); } 1: time: add "NOW() good" indicator 2: x86/Intel: split model-specific freq calculation off of intel_log_freq() 3: x86/time: avoid early uses of NOW() to return zero Jan printk_start_of_line() checks for a value of 0 right now. In order to be able to have NOW() return at least monotonically increasing values, that needs replacing by an explicit indicator. Signed-off-by: Jan Beulich <jbeulich@suse.com> --- Arm and RISC-V may want to consider whether their initial get_cycles() can't be moved yet earlier, such that the indicator also can be set yet earlier. --- v4: Add barriers. v3: New. --- a/xen/arch/arm/time.c +++ b/xen/arch/arm/time.c @@ -XXX,XX +XXX,XX @@ void __init preinit_xen_time(void) panic("Timer: Cannot initialize platform timer\n"); boot_count = get_cycles(); + smp_wmb(); + NOW_good = true; } static void __init init_dt_xen_time(void) --- a/xen/arch/riscv/time.c +++ b/xen/arch/riscv/time.c @@ -XXX,XX +XXX,XX @@ void __init preinit_xen_time(void) panic("%s: ACPI isn't supported\n", __func__); boot_clock_cycles = get_cycles(); + smp_wmb(); + NOW_good = true; /* set_xen_timer must have been set by sbi_init() already */ ASSERT(set_xen_timer); --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -XXX,XX +XXX,XX @@ void __init early_time_init(void) set_time_scale(&t->tsc_scale, tmp); t->stamp.local_tsc = boot_tsc_stamp; + NOW_good = true; init_percpu_time(); --- a/xen/common/time.c +++ b/xen/common/time.c @@ -XXX,XX +XXX,XX @@ #include <asm/div64.h> #include <asm/domain.h> +bool __ro_after_init NOW_good; + /* Nonzero if YEAR is a leap year (every 4 years, except every 100th isn't, and every 400th is). */ #define __isleap(year) \ --- a/xen/drivers/char/console.c +++ b/xen/drivers/char/console.c @@ -XXX,XX +XXX,XX @@ static void printk_start_of_line(const c } /* fall through */ case TSM_BOOT: - sec = NOW(); - nsec = do_div(sec, 1000000000); - - if ( sec | nsec ) + if ( NOW_good ) { + smp_rmb(); + sec = NOW(); + nsec = do_div(sec, 1000000000); snprintf(tstr, sizeof(tstr), "[%5"PRIu64".%06"PRIu64"] ", sec, nsec / 1000); break; --- a/xen/include/xen/time.h +++ b/xen/include/xen/time.h @@ -XXX,XX +XXX,XX @@ struct tm wallclock_time(uint64_t *ns); /* Chosen so (NOW() + delta) wont overflow without an uptime of 200 years */ #define STIME_DELTA_MAX ((s_time_t)((uint64_t)~0ULL>>2)) +/* + * Indicator that the value returned by NOW() is good (earlier invocations may + * return zero or very small, merely monotonically increasing values). + */ +extern bool NOW_good; + /* Explicitly OR with 1 just in case version number gets out of sync. */ #define version_update_begin(v) (((v) + 1) | 1) #define version_update_end(v) ((v) + 1) ..., for that logic to become reusable. While doing so undo the open- coding of DIV_ROUND_UP(). Also switch to the new struct cpuinfo_x86 field names. Signed-off-by: Jan Beulich <jbeulich@suse.com> --- If Misra didn't dislike non-static functions without external callers, the new function could be put below the old one, thus reducing churn and improving readability of the diff (really I moved the code for the new function up, but the diff representation is the other way around). --- v3: New. --- a/xen/arch/x86/cpu/intel.c +++ b/xen/arch/x86/cpu/intel.c @@ -XXX,XX +XXX,XX @@ static int num_cpu_cores(struct cpuinfo_ return 1; } -static void intel_log_freq(const struct cpuinfo_x86 *c) +static void intel_process_freq(const struct cpuinfo_x86 *c, + unsigned int *min_mhz, unsigned int *max_mhz) { - unsigned int eax, ebx, ecx, edx, factor; uint64_t msrval; uint8_t max_ratio, min_ratio; + unsigned int factor; - if ( c->cpuid_level >= 0x15 ) - { - cpuid(0x15, &eax, &ebx, &ecx, &edx); - if ( ecx && ebx && eax ) - { - unsigned long long val = ecx; - - val *= ebx; - printk("CPU%u: TSC: %u Hz * %u / %u = %Lu Hz\n", - smp_processor_id(), ecx, ebx, eax, val / eax); - } - else if ( ecx | eax | ebx ) - { - printk("CPU%u: TSC:", smp_processor_id()); - if ( ecx ) - printk(" core: %u Hz", ecx); - if ( ebx && eax ) - printk(" ratio: %u / %u", ebx, eax); - printk("\n"); - } - } - - if ( c->cpuid_level >= 0x16 ) - { - cpuid(0x16, &eax, &ebx, &ecx, &edx); - if ( ecx | eax | ebx ) - { - printk("CPU%u:", smp_processor_id()); - if ( ecx ) - printk(" bus: %u MHz", ecx); - if ( eax ) - printk(" base: %u MHz", eax); - if ( ebx ) - printk(" max: %u MHz", ebx); - printk("\n"); - } - } - - switch ( c->x86 ) + switch ( c->family ) { static const unsigned short core_factors[] = { 26667, 13333, 20000, 16667, 33333, 10000, 40000 }; @@ -XXX,XX +XXX,XX @@ static void intel_log_freq(const struct if ( !max_ratio ) return; - switch ( c->x86_model ) + switch ( c->model ) { case 0x0e: /* Core */ case 0x0f: case 0x16: case 0x17: case 0x1d: /* Core2 */ @@ -XXX,XX +XXX,XX @@ static void intel_log_freq(const struct return; } + if ( min_mhz ) + *min_mhz = DIV_ROUND_UP(factor * min_ratio, 100); + *max_mhz = DIV_ROUND_UP(factor * max_ratio, 100); +} + +static void intel_log_freq(const struct cpuinfo_x86 *c) +{ + unsigned int eax, ebx, ecx, edx, min_mhz = 0, max_mhz = 0; + + if ( c->cpuid_level >= 0x15 ) + { + cpuid(0x15, &eax, &ebx, &ecx, &edx); + if ( ecx && ebx && eax ) + { + unsigned long long val = ecx; + + val *= ebx; + printk("CPU%u: TSC: %u Hz * %u / %u = %Lu Hz\n", + smp_processor_id(), ecx, ebx, eax, val / eax); + } + else if ( ecx | eax | ebx ) + { + printk("CPU%u: TSC:", smp_processor_id()); + if ( ecx ) + printk(" core: %u Hz", ecx); + if ( ebx && eax ) + printk(" ratio: %u / %u", ebx, eax); + printk("\n"); + } + } + + if ( c->cpuid_level >= 0x16 ) + { + cpuid(0x16, &eax, &ebx, &ecx, &edx); + if ( ecx | eax | ebx ) + { + printk("CPU%u:", smp_processor_id()); + if ( ecx ) + printk(" bus: %u MHz", ecx); + if ( eax ) + printk(" base: %u MHz", eax); + if ( ebx ) + printk(" max: %u MHz", ebx); + printk("\n"); + } + } + + intel_process_freq(c, &min_mhz, &max_mhz); + if ( !max_mhz ) + return; + printk("CPU%u: ", smp_processor_id()); - if ( min_ratio ) - printk("%u ... ", (factor * min_ratio + 50) / 100); - printk("%u MHz\n", (factor * max_ratio + 50) / 100); + if ( min_mhz ) + printk("%u ... ", min_mhz); + printk("%u MHz\n", max_mhz); } static void init_intel_perf(struct cpuinfo_x86 *c) Waiting loops like the one in flush_command_buffer() will degenerate to infinite ones when used early enough for NOW() to still return constant zero. Make sure the returned value at least monotonically increases. When available, use nominal frequency values as initial approximation. Do this only in get_s_time(), as producing a sane value in get_s_time_fixed() for non-zero inputs won't be reasonably possible. Put an assertion there. Reported-by: Roger Pau Monné <roger.pau@citrix.com> Signed-off-by: Jan Beulich <jbeulich@suse.com> --- RFC: While generally the mentioned waiting loops will take longer to time out, on a very fast CPU tight loops may time out too early. RFC: On the 2nd pass through early_cpu_init() it may be okay to skip the new additions. With "x86/time: set AP's TSC scale estimate earlier" the counter update may not need to be atomic anymore, as then only the BSP can reasonably hit that path. I don't think Fixes: tags should be put here. If we did, we'd have to enumerate all introductions of early uses of NOW() (or get_s_time()), with the exception of those dealing with getting back 0 (which I expect is only printk_start_of_line()). Will want backporting nevertheless (unless deemed too risky). --- v3: Use "high" / "max" freq if "nominal" isn't available. Set NOW_good. v2: Add assertion to get_s_time_fixed(). Use nominal frequencies for very early setting, if available. --- a/xen/arch/x86/cpu/common.c +++ b/xen/arch/x86/cpu/common.c @@ -XXX,XX +XXX,XX @@ #include <asm/random.h> #include <asm/setup.h> #include <asm/shstk.h> +#include <asm/time.h> #include <asm/xstate.h> #include <public/sysctl.h> @@ -XXX,XX +XXX,XX @@ void __init early_cpu_init(bool verbose) &c->x86_capability[FEATURESET_7d1]); } + if (c->cpuid_level >= 0x15) { + cpuid(0x15, &eax, &ebx, &ecx, &edx); + + if (ecx && ebx && eax) + preset_tsc_scale(DIV_ROUND_UP(ecx * 1UL * ebx, eax)); + else if (c->cpuid_level >= 0x16) { + /* Assume CPU base freq ≈ TSC freq. */ + cpuid(0x16, &eax, &ebx, &ecx, &edx); + if (eax) + preset_tsc_scale(eax * 1000000UL); + else if (ebx) /* See preset_tsc_scale() for why. */ + preset_tsc_scale(ebx * 1000000UL); + } + } else if (c->vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) { + unsigned int nom_mhz = 0, hi_mhz = 0; + + amd_process_freq(c, NULL, &nom_mhz, &hi_mhz); + if (nom_mhz) + preset_tsc_scale(nom_mhz * 1000000UL); + else if (hi_mhz) /* See preset_tsc_scale() for why. */ + preset_tsc_scale(hi_mhz * 1000000UL); + } else if (c->vendor & X86_VENDOR_INTEL) { + unsigned int hi_mhz = 0; + + /* See preset_tsc_scale() for why. */ + intel_process_freq(c, NULL, &hi_mhz); + if (hi_mhz) + preset_tsc_scale(hi_mhz * 1000000UL); + } + eax = cpuid_eax(0x80000000); if ((eax >> 16) == 0x8000 && eax >= 0x80000008) { ebx = eax >= 0x8000001f ? cpuid_ebx(0x8000001f) : 0; --- a/xen/arch/x86/include/asm/time.h +++ b/xen/arch/x86/include/asm/time.h @@ -XXX,XX +XXX,XX @@ mktime (unsigned int year, unsigned int int time_suspend(void); int time_resume(void); +void preset_tsc_scale(unsigned long freq); void init_percpu_time(void); void time_latch_stamps(void); --- a/xen/arch/x86/cpu/intel.c +++ b/xen/arch/x86/cpu/intel.c @@ -XXX,XX +XXX,XX @@ static int num_cpu_cores(struct cpuinfo_ return 1; } -static void intel_process_freq(const struct cpuinfo_x86 *c, - unsigned int *min_mhz, unsigned int *max_mhz) +void intel_process_freq(const struct cpuinfo_x86 *c, + unsigned int *min_mhz, unsigned int *max_mhz) { uint64_t msrval; uint8_t max_ratio, min_ratio; --- a/xen/arch/x86/include/asm/processor.h +++ b/xen/arch/x86/include/asm/processor.h @@ -XXX,XX +XXX,XX @@ static inline uint8_t get_cpu_family(uin return fam; } +void intel_process_freq(const struct cpuinfo_x86 *c, + unsigned int *min_mhz, unsigned int *max_mhz); + #ifdef CONFIG_INTEL extern int8_t opt_tsx; extern bool rtm_disabled; --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -XXX,XX +XXX,XX @@ s_time_t get_s_time_fixed(uint64_t at_ts const struct cpu_time *t = &this_cpu(cpu_time); uint64_t tsc, delta; + /* scale_delta() degenerates when the scale wasn't set yet. */ + ASSERT(t->tsc_scale.mul_frac); + if ( at_tsc ) tsc = at_tsc; else @@ -XXX,XX +XXX,XX @@ s_time_t get_s_time_fixed(uint64_t at_ts s_time_t get_s_time(void) { + /* + * Before the TSC scale is set, avoid returning constant 0 (or whatever + * this_cpu(cpu_time).stamp.local_stime is set to). While the returned + * value is in no way representing time, it at least increases + * monotonically, thus avoiding e.g. waiting loops to degenerate to + * entirely infinite ones. + */ + if ( unlikely(!this_cpu(cpu_time).tsc_scale.mul_frac) ) + { + static s_time_t counter; + + return arch_fetch_and_add(&counter, 1); + } + return get_s_time_fixed(0); } @@ -XXX,XX +XXX,XX @@ int __init init_xen_time(void) return 0; } +/* BSP-only function to pre-set an approximate TSC scale. */ +void __init preset_tsc_scale(unsigned long freq) +{ + struct cpu_time *t = &this_cpu(cpu_time); + + /* + * The incoming frequency is only approximate (nominal). Increase it by + * 1% to make NOW() output rather a little too slow than too fast, thus + * avoiding a possible backwards jump once the final scale is set. + */ + freq += DIV_ROUND_UP(freq, 100); + + set_time_scale(&t->tsc_scale, freq); + t->stamp.local_tsc = boot_tsc_stamp; + NOW_good = true; +} /* Early init function. */ void __init early_time_init(void) @@ -XXX,XX +XXX,XX @@ void __init early_time_init(void) "TSC ADJUST set to %lx on boot CPU - clearing\n", tmp); wrmsrl(MSR_IA32_TSC_ADJUST, 0); boot_tsc_stamp -= tmp; + + if ( t->stamp.local_tsc ) + t->stamp.local_tsc -= tmp; } }