[PATCH v6 6/6] x86/hyperv: Fix serial console interrupts for TDX guests

[Dexuan Cui <decui@microsoft.com>]

When a TDX guest runs on Hyper-V, the UEFI firmware sets the HW_REDUCED
flag, and consequently ttyS0 interrupts can't work. Fix the issue by
overriding x86_init.acpi.reduced_hw_early_init().

Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
---

Changes since v1:
    None.

Changes in v5:
    Improved the comment [Michael Kelley]
    Added Michael's Reviewed-by.

Changes in v6: None.

 arch/x86/kernel/cpu/mshyperv.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index b95b689efa07..d642e3624286 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -313,6 +313,26 @@ static void __init hv_smp_prepare_cpus(unsigned int max_cpus)
 }
 #endif
 
+/*
+ * When a TDX guest runs on Hyper-V, the firmware sets the HW_REDUCED flag: see
+ * acpi_tb_create_local_fadt(). Consequently ttyS0 interrupts can't work because
+ * request_irq() -> ... -> irq_to_desc() returns NULL for ttyS0. This happens
+ * because mp_config_acpi_legacy_irqs() sees a nr_legacy_irqs() of 0, so it
+ * doesn't initialize the array 'mp_irqs[]', and later setup_IO_APIC_irqs() ->
+ * find_irq_entry() fails to find the legacy irqs from the array, and hence
+ * doesn't create the necessary irq description info.
+ *
+ * Clone arch/x86/kernel/acpi/boot.c: acpi_generic_reduced_hw_init() here,
+ * except don't change 'legacy_pic'. It keeps its default value
+ * 'default_legacy_pic'. mp_config_acpi_legacy_irqs() sees a non-zero
+ * nr_legacy_irqs(), and eventually serial console interrupts works properly.
+ */
+static void __init reduced_hw_init(void)
+{
+	x86_init.timers.timer_init	= x86_init_noop;
+	x86_init.irqs.pre_vector_init	= x86_init_noop;
+}
+
 static void __init ms_hyperv_init_platform(void)
 {
 	int hv_max_functions_eax;
@@ -424,6 +444,8 @@ static void __init ms_hyperv_init_platform(void)
 
 			/* A TDX VM must use x2APIC and doesn't use lazy EOI */
 			ms_hyperv.hints &= ~HV_X64_APIC_ACCESS_RECOMMENDED;
+
+			x86_init.acpi.reduced_hw_early_init = reduced_hw_init;
 		}
 	}
 
-- 
2.25.1