[PATCH V3 1/2] x86/tdx: Route safe halt execution via tdx_safe_halt()

[Vishal Annapurve]

Direct HLT instruction execution causes #VEs for TDX VMs which is routed
to hypervisor via TDCALL. safe_halt() routines execute HLT in STI-shadow
so IRQs need to remain disabled until the TDCALL to ensure that pending
IRQs are correctly treated as wake events. So "sti;hlt" sequence needs to
be replaced with "TDCALL; raw_local_irq_enable()" for TDX VMs.

Commit bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests")
prevented the idle routines from using "sti;hlt". But it missed the
paravirt routine which can be reached like this as an example:
        acpi_safe_halt() =>
        raw_safe_halt()  =>
        arch_safe_halt() =>
        irq.safe_halt()  =>
        pv_native_safe_halt()

Modify tdx_safe_halt() to implement the sequence "TDCALL;
raw_local_irq_enable()" and invoke tdx_halt() from idle routine which just
executes TDCALL without changing state of interrupts.

If CONFIG_PARAVIRT_XXL is disabled, "sti;hlt" sequences can still get
executed from TDX VMs via paths like:
        acpi_safe_halt() =>
        raw_safe_halt()  =>
        arch_safe_halt() =>
	native_safe_halt()
There is a long term plan to fix these paths by carving out
irq.safe_halt() outside paravirt framework.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests")
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>@linux.intel.com>
Signed-off-by: Vishal Annapurve <vannapurve@xxxxxxxxxx>
---
Changes since V2:
1) Addressed comments from Dave H and Kirill S.

V2: https://lore.kernel.org/lkml/20250129232525.3519586-1-vannapurve@xxxxxxxxxx/

 arch/x86/coco/tdx/tdx.c    | 18 +++++++++++++++++-
 arch/x86/include/asm/tdx.h |  2 +-
 arch/x86/kernel/process.c  |  2 +-
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 32809a06dab4..5e68758666a4 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -14,6 +14,7 @@
 #include <asm/ia32.h>
 #include <asm/insn.h>
 #include <asm/insn-eval.h>
+#include <asm/paravirt_types.h>
 #include <asm/pgtable.h>
 #include <asm/set_memory.h>
 #include <asm/traps.h>
@@ -398,7 +399,7 @@ static int handle_halt(struct ve_info *ve)
 	return ve_instr_len(ve);
 }
 
-void __cpuidle tdx_safe_halt(void)
+void __cpuidle tdx_halt(void)
 {
 	const bool irq_disabled = false;
 
@@ -409,6 +410,12 @@ void __cpuidle tdx_safe_halt(void)
 		WARN_ONCE(1, "HLT instruction emulation failed\n");
 }
 
+static void __cpuidle tdx_safe_halt(void)
+{
+	tdx_halt();
+	raw_local_irq_enable();
+}
+
 static int read_msr(struct pt_regs *regs, struct ve_info *ve)
 {
 	struct tdx_module_args args = {
@@ -1109,6 +1116,15 @@ void __init tdx_early_init(void)
 	x86_platform.guest.enc_kexec_begin	     = tdx_kexec_begin;
 	x86_platform.guest.enc_kexec_finish	     = tdx_kexec_finish;
 
+#ifdef CONFIG_PARAVIRT_XXL
+	/*
+	 * Avoid the literal hlt instruction in TDX guests. hlt will
+	 * induce a #VE in the STI-shadow which will enable interrupts
+	 * in a place where they are not wanted.
+	 */
+	pv_ops.irq.safe_halt = tdx_safe_halt;
+#endif
+
 	/*
 	 * TDX intercepts the RDMSR to read the X2APIC ID in the parallel
 	 * bringup low level code. That raises #VE which cannot be handled
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index b4b16dafd55e..393ee2dfaab1 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -58,7 +58,7 @@ void tdx_get_ve_info(struct ve_info *ve);
 
 bool tdx_handle_virt_exception(struct pt_regs *regs, struct ve_info *ve);
 
-void tdx_safe_halt(void);
+void tdx_halt(void);
 
 bool tdx_early_handle_ve(struct pt_regs *regs);
 
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 6da6769d7254..d11956a178df 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -934,7 +934,7 @@ void __init select_idle_routine(void)
 		static_call_update(x86_idle, mwait_idle);
 	} else if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
 		pr_info("using TDX aware idle routine\n");
-		static_call_update(x86_idle, tdx_safe_halt);
+		static_call_update(x86_idle, tdx_halt);
 	} else {
 		static_call_update(x86_idle, default_idle);
 	}
-- 
2.48.1.502.g6dc24dfdaf-goog