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Tue, 28 Jul 2026 04:37:16 -0700 (PDT) Message-ID: Date: Tue, 28 Jul 2026 13:37:15 +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-16d1c6/1785238637-1EEC477B-BBB15F33/0/0 X-purgate-type: clean X-purgate-size: 1372 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. Jan