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Tue, 06 Jan 2026 05:58:12 -0800 (PST) Message-ID: <1f539879-3083-41d5-a2c5-c63c9161f0bf@suse.com> Date: Tue, 6 Jan 2026 14:58:11 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH 1/5] x86/time: deal with negative deltas in get_s_time_fixed() From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , =?UTF-8?B?0JDQvdGC0L7QvSDQnNCw0YDQutC+0LI=?= References: <66a53368-9c33-436c-858e-2b2d25ae84b7@suse.com> 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: <66a53368-9c33-436c-858e-2b2d25ae84b7@suse.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1767707910399158500 Callers may pass in TSC values from before the local TSC stamp was last updated (this would in particular be the case when the TSC was latched, a time rendezvous occurs, and the latched value is used only afterwards). scale_delta(), otoh, deals with unsigned values, and hence would treat negative incoming deltas as huge positive values, yielding entirely bogus return values. Fixes: 88e64cb785c1 ("x86/HVM: use fixed TSC value when saving or restoring= domain") Reported-by: =D0=90=D0=BD=D1=82=D0=BE=D0=BD =D0=9C=D0=B0=D1=80=D0=BA=D0=BE= =D0=B2 Signed-off-by: Jan Beulich Reviewed-by: Roger Pau Monn=C3=A9 --- An alternative might be to have scale_delta() deal with signed deltas, yet that seemed more risky to me. There could likely be more Fixes: tags; the one used is the oldest applicable one, from what I can tell. A similar issue looks to exist in read_xen_timer() and its read_cycle() helper, if we're scheduled out (and beck in) between reading of the TSC and calculation of the delta (involving ->tsc_timestamp). Am I overlooking anything there? stime2tsc() guards against negative deltas by using 0 instead; I'm not quite sure that's correct either. amd_check_erratum_1474() (next to its call to tsc_ticks2ns()) has a comment towards the TSC being "sane", but is that correct? Due to TSC_ADJUST, rdtsc() may well return a huge value (and the TSC would then wrap through 0 at some point). Shouldn't we subtract boot_tsc_stamp before calling tsc_ticks2ns()? A similar issue looks to exist in tsc_get_info(), again when rdtsc() possibly returns a huge value due to TSC_ADJUST. Once again I wonder whether we shouldn't subtract boot_tsc_stamp. --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -1649,8 +1649,13 @@ s_time_t get_s_time_fixed(u64 at_tsc) tsc =3D at_tsc; else tsc =3D rdtsc_ordered(); - delta =3D tsc - t->stamp.local_tsc; - return t->stamp.local_stime + scale_delta(delta, &t->tsc_scale); + + if ( tsc >=3D t->stamp.local_tsc ) + delta =3D scale_delta(tsc - t->stamp.local_tsc, &t->tsc_scale); + else + delta =3D -scale_delta(t->stamp.local_tsc - tsc, &t->tsc_scale); + + return t->stamp.local_stime + delta; } =20 s_time_t get_s_time(void)