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Wed, 29 Jul 2026 01:57:13 -0700 (PDT) Message-ID: Date: Wed, 29 Jul 2026 10:57:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/3] time: add "NOW() good" indicator To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Julien Grall , Stefano Stabellini , Anthony PERARD , Michal Orzel , Teddy Astie , Bertrand Marquis , Volodymyr Babchuk , Oleksii Kurochko References: Content-Language: en-US From: Jan Beulich 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-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-purgate-ID: tlsNG-4011c0/1785315433-50CCBCFC-A56F712B/0/0 X-purgate-type: clean X-purgate-size: 1798 On 29.07.2026 10:46, Roger Pau Monné wrote: > On Tue, Jul 28, 2026 at 01:37:15PM +0200, Jan Beulich wrote: >> On 28.07.2026 12:32, Roger Pau Monné wrote: >>> On Tue, Jul 28, 2026 at 10:24:38AM +0200, Jan Beulich wrote: >>>> On 28.07.2026 10:17, Roger Pau Monné wrote: >>>>> On Tue, Jun 30, 2026 at 04:06:00PM +0200, Jan Beulich wrote: >>>>>> --- a/xen/arch/x86/time.c >>>>>> +++ b/xen/arch/x86/time.c >>>>>> @@ -2660,6 +2660,7 @@ void __init early_time_init(void) >>>>>> >>>>>> set_time_scale(&t->tsc_scale, tmp); >>>>>> t->stamp.local_tsc = boot_tsc_stamp; >>>>>> + NOW_good = true; >>>>> >>>>> Would you need a barrier here to ensure compiler doesn't re-order the >>>>> writes? Maybe using ACCESS_ONCE(), or a smp_wmb() like it's used in >>>>> other arches? >>>> >>>> Aiui it isn't needed here. The compiler can't re-order the two writes, >>>> due to our use of -fno-strict-aliasing. The barrier is there on other >>>> arch-es to avoid re-ordering in hardware. >>> >>> You are the expert in compilers, but it was my understanding that >>> `no-strict-aliasing` only affects the ordering of pointers accesses, >>> but not plain variables. IOW: NOW_good accesses could be reordered >>> because it's not a pointer. Does the compiler consider t can point to >>> &NOW_good and hence it can't be reordered? >> >> Yes, that's my understanding of how this work. (Yet no, "expert" surely >> is going too far.) Things would be different if NOW_good was a function- >> local variable, I think. > > OK, I wouldn't mind adding a compiler barrier to say on the safe side, > but I also won't object if you think that's enough. Okay, I've added one in, even if it feels a little odd. > Acked-by: Roger Pau Monné Thanks, Jan