[PATCH v3 13/13] x86/hyperv/vtl: Use the wakeup mailbox to boot secondary CPUs

Ricardo Neri
2025-05-03T19:10:20Z

The hypervisor is an untrusted entity for TDX guests. It cannot be used
to boot secondary CPUs. The function hv_vtl_wakeup_secondary_cpu() cannot
be used.

Instead, the virtual firmware boots the secondary CPUs and places them in
a state to transfer control to the kernel using the wakeup mailbox.

The kernel updates the APIC callback wakeup_secondary_cpu_64() to use
the mailbox if detected early during boot (enumerated via either an ACPI
table or a DeviceTree node).

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Changes since v2:
- Unconditionally use the wakeup mailbox in a TDX confidential VM.
(Michael).
- Edited the commit message for clarity.

Changes since v1:
- None
---
arch/x86/hyperv/hv_vtl.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index cd48bedd21f0..30a5a0c156c1 100644
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -299,7 +299,15 @@ int __init hv_vtl_early_init(void)
panic("XSAVE has to be disabled as it is not supported by this module.\n"
"Please add 'noxsave' to the kernel command line.\n");

- apic_update_callback(wakeup_secondary_cpu_64, hv_vtl_wakeup_secondary_cpu);
+ /*
+ * TDX confidential VMs do not trust the hypervisor and cannot use it to
+ * boot secondary CPUs. Instead, they will be booted using the wakeup
+ * mailbox if detected during boot. See setup_arch().
+ *
+ * There is no paravisor present if we are here.
+ */
+ if (!hv_isolation_type_tdx())
+ apic_update_callback(wakeup_secondary_cpu_64, hv_vtl_wakeup_secondary_cpu);

return 0;
}
--
2.43.0