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In addition, >>> with commit 38669ba205d1 ("x86/xen/time: Output xen sched_clock time from >>> 0"), kdump kernel in Xen HVM guest may panic at very early stage when >>> accessing &__this_cpu_read(xen_vcpu)->time as in below: >>> >>> setup_arch() >>>   -> init_hypervisor_platform() >>>       -> x86_init.hyper.init_platform = xen_hvm_guest_init() >>>           -> xen_hvm_init_time_ops() >>>               -> xen_clocksource_read() >>>                   -> src = &__this_cpu_read(xen_vcpu)->time; >>> >>> This is because Xen HVM supports at most MAX_VIRT_CPUS=32 'vcpu_info' >>> embedded inside 'shared_info' during early stage until xen_vcpu_setup() is >>> used to allocate/relocate 'vcpu_info' for boot cpu at arbitrary address. >>> >>> However, when Xen HVM guest panic on vcpu >= 32, since >>> xen_vcpu_info_reset(0) would set per_cpu(xen_vcpu, cpu) = NULL when >>> vcpu >= 32, xen_clocksource_read() on vcpu >= 32 would panic. >>> >>> This patch delays xen_hvm_init_time_ops() to later in >>> xen_hvm_smp_prepare_boot_cpu() after the 'vcpu_info' for boot vcpu is >>> registered when the boot vcpu is >= 32. >>> >>> Another option is to always delay xen_hvm_init_time_ops() for any vcpus >>> (including vcpu=0). Since to delay xen_hvm_init_time_ops() may lead to >>> clock backward issue, >> >> This is referring to >> https://lists.xenproject.org/archives/html/xen-devel/2021-10/msg01516.html I >> assume? > No. > > So far there are clock forward (e.g., from 0 to 65345) issue and clock backward > issue (e.g., from 2.432 to 0). > > The clock forward issue can be resolved by above link to enforce clock update > during vcpu info registration. However, so far I am only able to reproduce clock > forward when "taskset -c 33 echo c > /proc/sysrq-trigger". > > That is, I am not able to see any clock forward drift during regular boot (on > CPU=0), without the fix at hypervisor side. > > The clock backward issue is because the native clock source is used if we delay > the initialization of xen clock source. We will see a backward when the source > is switched from native to xen. > >> >>>   it is preferred to avoid that for regular boot (The >>> pv_sched_clock=native_sched_clock() is used at the very beginning until >>> xen_sched_clock() is registered). That requires to adjust >>> xen_sched_clock_offset. That's why we only delay xen_hvm_init_time_ops() >>> for vcpu>=32. >> >> We delay only on VCPU>=32 because we want to avoid the clock going backwards due >> to hypervisor problem pointed to be the link above, not because we need to >> adjust xen_sched_clock_offset (which we could if we wanted). > I will add that. > > Just to clarify that so far I am not able to reproduce the clock backward issue > during regular boot (on CPU=0), when the fix is not available at hypervisor side. FTR, Dongli and I had a chat and he is going to provide another version of the patch where time initialization is deferred for all vcpus. -boris