linux-kernel - [PATCH v4 10/10] x86/hyperv/vtl: Use the wakeup mailbox to boot secondary CPUs

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Message-Id: <20250603-rneri-wakeup-mailbox-v4-10-d533272b7232@linux.intel.com>
Date: Tue, 03 Jun 2025 17:15:22 -0700
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: x86@...nel.org, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
 Conor Dooley <conor+dt@...nel.org>, Rob Herring <robh@...nel.org>, 
 "K. Y. Srinivasan" <kys@...rosoft.com>, 
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
 Dexuan Cui <decui@...rosoft.com>, Michael Kelley <mhklinux@...look.com>, 
 "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Saurabh Sengar <ssengar@...ux.microsoft.com>, 
 Chris Oo <cho@...rosoft.com>, 
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
 linux-hyperv@...r.kernel.org, devicetree@...r.kernel.org, 
 linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org, 
 "Ravi V. Shankar" <ravi.v.shankar@...el.com>, 
 Ricardo Neri <ricardo.neri@...el.com>, 
 Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Subject: [PATCH v4 10/10] x86/hyperv/vtl: Use the wakeup mailbox to boot
 secondary CPUs

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@...look.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.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(-)

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;
 }

-- 
2.43.0