[PATCH v3 12/13] x86/hyperv/vtl: Mark the wakeup mailbox page as private

Ricardo Neri
2025-05-03T19:10:19Z

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

The current code maps MMIO devices as shared (decrypted) by default in a
confidential computing VM.

In a TDX environment, secondary CPUs are booted using the Multiprocessor
Wakeup Structure defined in the ACPI specification. The virtual firmware
and the operating system function in the guest context, without
intervention from the VMM. Map the physical memory of the mailbox as
private. Use the is_private_mmio() callback.

Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Changes since v2:
- Use the new helper function get_mp_wakeup_mailbox_paddr().
- Edited the commit message for clarity.

Changes since v1:
- Added the helper function within_page() to improve readability
- Override the is_private_mmio() callback when detecting a TDX
environment. The address of the mailbox is checked in
hv_is_private_mmio_tdx().
---
arch/x86/hyperv/hv_vtl.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index 8b497c8292d3..cd48bedd21f0 100644
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -54,6 +54,18 @@ static void __noreturn hv_vtl_restart(char __maybe_unused *cmd)
hv_vtl_emergency_restart();
}

+static inline bool within_page(u64 addr, u64 start)
+{
+ return addr >= start && addr < (start + PAGE_SIZE);
+}
+
+static bool hv_vtl_is_private_mmio_tdx(u64 addr)
+{
+ u64 mb_addr = get_mp_wakeup_mailbox_paddr();
+
+ return mb_addr && within_page(addr, mb_addr);
+}
+
void __init hv_vtl_init_platform(void)
{
pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
@@ -61,6 +73,8 @@ void __init hv_vtl_init_platform(void)
/* There is no paravisor present if we are here. */
if (hv_isolation_type_tdx()) {
x86_init.resources.realmode_limit = SZ_4G;
+ x86_platform.hyper.is_private_mmio = hv_vtl_is_private_mmio_tdx;
+
} else {
x86_platform.realmode_reserve = x86_init_noop;
x86_platform.realmode_init = x86_init_noop;
--
2.43.0