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[v4,10/10] x86/hyperv/vtl: Use the wakeup mailbox to boot secondary CPUs

Message ID 20250603-rneri-wakeup-mailbox-v4-10-d533272b7232@linux.intel.com
State New
Headers show
Series x86/hyperv/hv_vtl: Use a wakeup mailbox to boot secondary CPUs | expand

Commit Message

Ricardo Neri June 4, 2025, 12:15 a.m. UTC 
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).

Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Changes since v3:
 - Added Reviewed-by tag from Michael. Thanks!

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(-)
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Patch

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index 995d1de7a9be..f3d4f06d1f17 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;
 }