Re: [patch 20/48] x86/apic: Enable TSC coupled programming mode

[Nathan Chancellor]

Hi Thomas,

On Tue, Feb 24, 2026 at 05:36:49PM +0100, Thomas Gleixner wrote:
> The TSC deadline timer is directly coupled to the TSC and setting the next
> deadline is tedious as the clockevents core code converts the
> CLOCK_MONOTONIC based absolute expiry time to a relative expiry by reading
> the current time from the TSC. It converts that delta to cycles and hands
> the result to lapic_next_deadline(), which then has read to the TSC and add
> the delta to program the timer.
> 
> The core code now supports coupled clock event devices and can provide the
> expiry time in TSC cycles directly without reading the TSC at all.
> 
> This obviouly works only when the TSC is the current clocksource, but
> that's the default for all modern CPUs which implement the TSC deadline
> timer. If the TSC is not the current clocksource (e.g. early boot) then the
> core code falls back to the relative set_next_event() callback as before.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx

After this change landed in -next as commit f246ec3478cf ("x86/apic:
Enable TSC coupled programming mode"), two of my Intel-based test
machines fail to boot. Unfortunately, I do not think I have any serial
access on these, so I have little introspective ability. Is there any
information I can provide or patches I can test to try and help figure
out what is going on here? I have attached the output of lscpu of both
machines, in case there is some common thread there.

Cheers,
Nathan

# bad: [d517cb8cea012f43b069617fc8179b45404f8018] Add linux-next specific files for 20260302
# good: [11439c4635edd669ae435eec308f4ab8a0804808] Linux 7.0-rc2
git bisect start 'd517cb8cea012f43b069617fc8179b45404f8018' '11439c4635edd669ae435eec308f4ab8a0804808'
# good: [30cad5d4db9212a3e9bb99be1d99c4fbc17966c7] Merge branch 'master' of https://git.kernel.org/pub/scm/linux/kernel/git/wpan/wpan-next.git
git bisect good 30cad5d4db9212a3e9bb99be1d99c4fbc17966c7
# good: [5add127981db7fda704fb251de1a3a77e3282e37] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git
git bisect good 5add127981db7fda704fb251de1a3a77e3282e37
# bad: [49b56e8a6ca8c20f7d9bb8904d4b2a5bf032554a] Merge branch 'char-misc-next' of https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
git bisect bad 49b56e8a6ca8c20f7d9bb8904d4b2a5bf032554a
# good: [1695e1c2db4247e8428badf667900beec51c5174] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git
git bisect good 1695e1c2db4247e8428badf667900beec51c5174
# bad: [c50f05bd3c4e992c1dfb61b14d6f7d999f1381f9] Merge branch into tip/master: 'sched/hrtick'
git bisect bad c50f05bd3c4e992c1dfb61b14d6f7d999f1381f9
# bad: [343f2f4dc5425107d509d29e26ef59c2053aeaa4] hrtimer: Try to modify timers in place
git bisect bad 343f2f4dc5425107d509d29e26ef59c2053aeaa4
# bad: [6abfc2bd5b0cff70db99a273f2a161e2273eae6d] hrtimer: Use guards where appropriate
git bisect bad 6abfc2bd5b0cff70db99a273f2a161e2273eae6d
# good: [0abec32a6836eca6b61ae81e4829f94abd4647c7] sched/hrtick: Mark hrtick timer LAZY_REARM
git bisect good 0abec32a6836eca6b61ae81e4829f94abd4647c7
# good: [23028286128d817a414eee0c0a2c6cdc57a83e6f] x86/apic: Avoid the PVOPS indirection for the TSC deadline timer
git bisect good 23028286128d817a414eee0c0a2c6cdc57a83e6f
# bad: [f246ec3478cfdab830ee0815209f48923e7ee5e2] x86/apic: Enable TSC coupled programming mode
git bisect bad f246ec3478cfdab830ee0815209f48923e7ee5e2
# good: [89f951a1e8ad781e7ac70eccddab0e0c270485f9] clockevents: Provide support for clocksource coupled comparators
git bisect good 89f951a1e8ad781e7ac70eccddab0e0c270485f9
# first bad commit: [f246ec3478cfdab830ee0815209f48923e7ee5e2] x86/apic: Enable TSC coupled programming mode
Architecture:                            x86_64
CPU op-mode(s):                          32-bit, 64-bit
Address sizes:                           39 bits physical, 48 bits virtual
Byte Order:                              Little Endian
CPU(s):                                  16
On-line CPU(s) list:                     0-15
Vendor ID:                               GenuineIntel
Model name:                              11th Gen Intel(R) Core(TM) i7-11700 @ 2.50GHz
CPU family:                              6
Model:                                   167
Thread(s) per core:                      2
Core(s) per socket:                      8
Socket(s):                               1
Stepping:                                1
CPU(s) scaling MHz:                      30%
CPU max MHz:                             4900.0000
CPU min MHz:                             800.0000
BogoMIPS:                                4992.00
Flags:                                   fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mpx avx512f avx512dq rdseed adx smap avx512ifma clflushopt intel_pt avx512cd sha_ni avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp hwp_pkg_req vnmi avx512vbmi umip pku ospke avx512_vbmi2 gfni vaes vpclmulqdq avx512_vnni avx512_bitalg avx512_vpopcntdq rdpid fsrm md_clear flush_l1d arch_capabilities
Virtualization:                          VT-x
L1d cache:                               384 KiB (8 instances)
L1i cache:                               256 KiB (8 instances)
L2 cache:                                4 MiB (8 instances)
L3 cache:                                16 MiB (1 instance)
NUMA node(s):                            1
NUMA node0 CPU(s):                       0-15
Vulnerability Gather data sampling:      Mitigation; Microcode
Vulnerability Ghostwrite:                Not affected
Vulnerability Indirect target selection: Mitigation; Aligned branch/return thunks
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Old microcode:             Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Mitigation; Enhanced IBRS
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1:                Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; Enhanced / Automatic IBRS; IBPB conditional; PBRSB-eIBRS SW sequence; BHI SW loop, KVM SW loop
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Not affected
Architecture:                            x86_64
CPU op-mode(s):                          32-bit, 64-bit
Address sizes:                           39 bits physical, 48 bits virtual
Byte Order:                              Little Endian
CPU(s):                                  4
On-line CPU(s) list:                     0-3
Vendor ID:                               GenuineIntel
Model name:                              Intel(R) N100
CPU family:                              6
Model:                                   190
Thread(s) per core:                      1
Core(s) per socket:                      4
Socket(s):                               1
Stepping:                                0
CPU(s) scaling MHz:                      41%
CPU max MHz:                             3400.0000
CPU min MHz:                             700.0000
BogoMIPS:                                1612.80
Flags:                                   fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l2 cdp_l2 ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdt_a rdseed adx smap clflushopt clwb intel_pt sha_ni xsaveopt xsavec xgetbv1 xsaves split_lock_detect user_shstk avx_vnni dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp hwp_pkg_req vnmi umip pku ospke waitpkg gfni vaes vpclmulqdq rdpid movdiri movdir64b fsrm md_clear serialize arch_lbr ibt flush_l1d arch_capabilities
Virtualization:                          VT-x
L1d cache:                               128 KiB (4 instances)
L1i cache:                               256 KiB (4 instances)
L2 cache:                                2 MiB (1 instance)
L3 cache:                                6 MiB (1 instance)
NUMA node(s):                            1
NUMA node0 CPU(s):                       0-3
Vulnerability Gather data sampling:      Not affected
Vulnerability Ghostwrite:                Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Old microcode:             Not affected
Vulnerability Reg file data sampling:    Mitigation; Clear Register File
Vulnerability Retbleed:                  Not affected
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1:                Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; Enhanced / Automatic IBRS; IBPB conditional; PBRSB-eIBRS Not affected; BHI BHI_DIS_S
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Mitigation; IBPB before exit to userspace