On 10/2/25 14:42, Wei Liu wrote:
> On Tue, Sep 23, 2025 at 02:46:08PM -0700, Mukesh Rathor wrote:
> [...]
>> +
>> +/*
>> + * This is the C entry point from the asm glue code after the devirt
>> hypercall.
>
> devirt -> devirtualization
>
>> + * We enter here in IA32-e long mode, ie, full 64bit mode running on kernel
>> + * page tables with our below 4G page identity mapped, but using a temporary
>> + * GDT. ds/fs/gs/es are null. ss is not usable. bp is null. stack is not
>> + * available. We restore kernel GDT, and rest of the context, and continue
>> + * to kexec.
>> + */
> [...]
>> +
>> +static noinline __noclone void crash_nmi_callback(struct pt_regs *regs)
>> +{
>> + struct hv_input_disable_hyp_ex *input;
>> + u64 status;
>> + int msecs = 1000, ccpu = smp_processor_id();
>> +
>> + if (ccpu == 0) {
>> + /* crash_save_cpu() will be done in the kexec path */
>> + cpu_emergency_stop_pt(); /* disable performance trace */
>> + atomic_inc(&crash_cpus_wait);
>> + } else {
>> + crash_save_cpu(regs, ccpu);
>> + cpu_emergency_stop_pt(); /* disable performance trace */
>> + atomic_inc(&crash_cpus_wait);
>> + for (;;)
>> + cpu_relax();
>> + }
>> +
>> + while (atomic_read(&crash_cpus_wait) < num_online_cpus() && msecs--)
>> + mdelay(1);
>> +
>> + stop_nmi();
>> + if (!hv_has_crashed)
>> + hv_notify_prepare_hyp();
>> +
>> + if (crashing_cpu == -1)
>> + crashing_cpu = ccpu; /* crash cmd uses this */
>> +
>> + hv_hvcrash_ctxt_save();
>> + hv_mark_tss_not_busy();
>> + hv_crash_fixup_kernpt();
>> +
>> + input = *this_cpu_ptr(hyperv_pcpu_input_arg);
>> + memset(input, 0, sizeof(*input));
>> + input->rip = trampoline_pa;
>> + input->arg = devirt_arg;
>> +
>> + status = hv_do_hypercall(HVCALL_DISABLE_HYP_EX, input, NULL);
>> +
>
> If I understand this correctly, after this call, upon return from the
> hypervisor, Linux will start executing the trampoline code.
correct.
>> + hv_panic_timeout_reboot();
>
> Why is this needed? Is it to catch the case when the hypercall fails?
correct.
> [...]
>> +static void __noclone hv_crash_stop_other_cpus(void)
>> +{
>> + static bool crash_stop_done;
>> + struct pt_regs lregs;
>> + int ccpu = smp_processor_id();
>> +
>> + if (hv_has_crashed)
>> + return; /* all cpus already in NMI handler path */
>> +
>> + if (!kexec_crash_loaded()) {
>> + hv_notify_prepare_hyp();
>> + hv_panic_timeout_reboot(); /* no return */
>> + }
>> +
>> + /* If hyp crashes also, we could come here again before cpus_stopped is
>
> hypervisor or hv (given the same term is used in the function)
>
>> + * set in crash_smp_send_stop(). So use our own check.
>> + */
>> + if (crash_stop_done)
>> + return;
>> + crash_stop_done = true;
>> +
>> + /* Linux has crashed: hv is healthy, we can ipi safely */
>
> IPI.
>
>> +
>> +err_out:
>> + unregister_nmi_handler(NMI_LOCAL, "hv_crash_nmi");
>> + pr_err("Hyper-V: only linux (but not hyp) kdump support enabled\n");
>
> hypervisor not hyp. This is a message for the user so we should be as
> clear as possible.
> Wei
>
>> +}
>> --
>> 2.36.1.vfs.0.0
>>
