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    SubjectRe: [PATCH v5 02/22] cc_platform: Add new attribute to prevent ACPI CPU hotplug
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    On Tue, 2022-07-19 at 10:46 -0700, Dave Hansen wrote:
    > On 7/13/22 04:09, Kai Huang wrote:
    > ...
    > > "TDX doesn’t support adding or removing CPUs from TDX security perimeter. The
    > > BIOS should prevent CPUs from being hot-added or hot-removed after platform
    > > boots."
    >
    > That's a start. It also probably needs to say that the security
    > perimeter includes all logical CPUs, though.

    To me it is kinda implied. But I have sent email to TDX spec owner to see
    whether we can say it more explicitly.

    >
    > > static int acpi_map_cpu2node(acpi_handle handle, int cpu, int physid)
    > > {
    > > @@ -819,6 +820,12 @@ int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid,
    > > u32 acpi_id,
    > > {
    > > int cpu;
    > >
    > > + if (platform_tdx_enabled()) {
    > > + pr_err("BIOS bug: CPU (physid %u) hot-added on TDX enabled
    > > platform. Reject it.\n",
    > > + physid);
    > > + return -EINVAL;
    > > + }
    >
    > Is this the right place? There are other sanity checks in
    > acpi_processor_hotadd_init() and it seems like a better spot.

    It has below additional check:

    if (invalid_phys_cpuid(pr->phys_id))
    return -ENODEV;

    status = acpi_evaluate_integer(pr->handle, "_STA", NULL, &sta);
    if (ACPI_FAILURE(status) || !(sta & ACPI_STA_DEVICE_PRESENT))
    return -ENODEV;


    I don't know exactly when will the first "invalid_phys_cpuid()" case happen, but
    the CPU is enumerated as "present" only after the second check. I.e. if BIOS is
    buggy and somehow sends a ACPI CPU hot-add event to kernel w/o having the CPU
    being actually hot-added, the kernel just returns -ENODEV here.

    So to me, adding to acpi_map_cpu() is more reasonable, because by reaching here,
    it is sure that a real CPU is being hot-added.


    >
    > > cpu = acpi_register_lapic(physid, acpi_id, ACPI_MADT_ENABLED);
    > > if (cpu < 0) {
    > > pr_info("Unable to map lapic to logical cpu number\n");
    > > @@ -835,6 +842,10 @@ EXPORT_SYMBOL(acpi_map_cpu);
    > >
    > > int acpi_unmap_cpu(int cpu)
    > > {
    > > + if (platform_tdx_enabled())
    > > + pr_err("BIOS bug: CPU %d hot-removed on TDX enabled platform.
    > > TDX is broken. Please reboot the machine.\n",
    > > + cpu);
    > > +
    > > #ifdef CONFIG_ACPI_NUMA
    > > set_apicid_to_node(per_cpu(x86_cpu_to_apicid, cpu), NUMA_NO_NODE);
    > > #endif
    >

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