[PATCH v3 10/13] x86/hyperv/vtl: Setup the 64-bit trampoline for TDX guests

Ricardo Neri
2025-05-03T19:10:17Z

From: Yunhong Jiang <yunhong.jiang@linux.intel.com>

The hypervisor is an untrusted entity for TDX guests. It cannot be used
to boot secondary CPUs - neither via hypercalls not the INIT assert,
de-assert plus Start-Up IPI messages.

Instead, the platform virtual firmware boots the secondary CPUs and
puts them in a state to transfer control to the kernel. This mechanism uses
the wakeup mailbox described in the Multiprocessor Wakeup Structure of the
ACPI specification. The entry point to the kernel is trampoline_start64.

Allocate and setup the trampoline using the default x86_platform callbacks.

The platform firmware configures the secondary CPUs in long mode. It is no
longer necessary to locate the trampoline under 1MB memory. After handoff
from firmware, the trampoline code switches briefly to 32-bit addressing
mode, which has an addressing limit of 4GB. Set the upper bound of the
trampoline memory accordingly.

Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Changes since v2:
- Added a note regarding there is no need to check for a present
paravisor.
- Edited commit message for clarity.

Changes since v1:
- Dropped the function hv_reserve_real_mode(). Instead, used the new
members realmode_limit and reserve_bios members of x86_init to
set the upper bound of the trampoline memory. (Thomas)
---
arch/x86/hyperv/hv_vtl.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index 6bd183ee484f..8b497c8292d3 100644
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -58,9 +58,14 @@ void __init hv_vtl_init_platform(void)
{
pr_info("Linux runs in Hyper-V Virtual Trust Level\n");

- x86_platform.realmode_reserve = x86_init_noop;
- x86_platform.realmode_init = x86_init_noop;
- real_mode_header = &hv_vtl_real_mode_header;
+ /* There is no paravisor present if we are here. */
+ if (hv_isolation_type_tdx()) {
+ x86_init.resources.realmode_limit = SZ_4G;
+ } else {
+ x86_platform.realmode_reserve = x86_init_noop;
+ x86_platform.realmode_init = x86_init_noop;
+ real_mode_header = &hv_vtl_real_mode_header;
+ }
x86_init.irqs.pre_vector_init = x86_init_noop;
x86_init.timers.timer_init = x86_init_noop;
x86_init.resources.probe_roms = x86_init_noop;
--
2.43.0