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Tue, 30 Jun 2026 07:06:01 -0700 (PDT) Message-ID: Date: Tue, 30 Jun 2026 16:06:00 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH v4 1/3] time: add "NOW() good" indicator From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , Julien Grall , Stefano Stabellini , Anthony PERARD , Michal Orzel , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Teddy Astie , Bertrand Marquis , Volodymyr Babchuk , Oleksii Kurochko References: Content-Language: en-US Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Transfer-Encoding: quoted-printable X-purgate-ID: tlsNG-d25034/1782828362-0EB3DD51-2E6F3598/0/0 X-purgate-type: clean X-purgate-size: 2820 X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1782828379781158500 Content-Type: text/plain; charset="utf-8" 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 Acked-by: Roger Pau Monn=C3=A9 Reviewed-by: Michal Orzel --- 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 @@ -145,6 +145,8 @@ void __init preinit_xen_time(void) panic("Timer: Cannot initialize platform timer\n"); =20 boot_count =3D get_cycles(); + smp_wmb(); + NOW_good =3D true; } =20 static void __init init_dt_xen_time(void) --- a/xen/arch/riscv/time.c +++ b/xen/arch/riscv/time.c @@ -87,6 +87,8 @@ void __init preinit_xen_time(void) panic("%s: ACPI isn't supported\n", __func__); =20 boot_clock_cycles =3D get_cycles(); + smp_wmb(); + NOW_good =3D true; =20 /* 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 @@ -2660,6 +2660,7 @@ void __init early_time_init(void) =20 set_time_scale(&t->tsc_scale, tmp); t->stamp.local_tsc =3D boot_tsc_stamp; + NOW_good =3D true; =20 init_percpu_time(); =20 --- a/xen/common/time.c +++ b/xen/common/time.c @@ -22,6 +22,8 @@ #include #include =20 +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 @@ -975,11 +975,11 @@ static void printk_start_of_line(const c } /* fall through */ case TSM_BOOT: - sec =3D NOW(); - nsec =3D do_div(sec, 1000000000); - - if ( sec | nsec ) + if ( NOW_good ) { + smp_rmb(); + sec =3D NOW(); + nsec =3D 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 @@ -62,6 +62,12 @@ 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)) =20 +/* + * 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)