[PATCH v2 22/44] x86/tdx: Remove TDX_HCALL_ISSUE_STI

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

Now that arch_cpu_idle() is expected to return with IRQs disabled,
avoid the useless STI/CLI dance.

Per the specs this is supposed to work, but nobody has yet relied up
this behaviour so broken implementations are possible.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/coco/tdx/tdcall.S        |   13 -------------
 arch/x86/coco/tdx/tdx.c           |   23 ++++-------------------
 arch/x86/include/asm/shared/tdx.h |    1 -
 3 files changed, 4 insertions(+), 33 deletions(-)

--- a/arch/x86/coco/tdx/tdcall.S
+++ b/arch/x86/coco/tdx/tdcall.S
@@ -139,19 +139,6 @@ SYM_FUNC_START(__tdx_hypercall)
 
 	movl $TDVMCALL_EXPOSE_REGS_MASK, %ecx
 
-	/*
-	 * For the idle loop STI needs to be called directly before the TDCALL
-	 * that enters idle (EXIT_REASON_HLT case). STI instruction enables
-	 * interrupts only one instruction later. If there is a window between
-	 * STI and the instruction that emulates the HALT state, there is a
-	 * chance for interrupts to happen in this window, which can delay the
-	 * HLT operation indefinitely. Since this is the not the desired
-	 * result, conditionally call STI before TDCALL.
-	 */
-	testq $TDX_HCALL_ISSUE_STI, %rsi
-	jz .Lskip_sti
-	sti
-.Lskip_sti:
 	tdcall
 
 	/*
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -169,7 +169,7 @@ static int ve_instr_len(struct ve_info *
 	}
 }
 
-static u64 __cpuidle __halt(const bool irq_disabled, const bool do_sti)
+static u64 __cpuidle __halt(const bool irq_disabled)
 {
 	struct tdx_hypercall_args args = {
 		.r10 = TDX_HYPERCALL_STANDARD,
@@ -189,20 +189,14 @@ static u64 __cpuidle __halt(const bool i
 	 * can keep the vCPU in virtual HLT, even if an IRQ is
 	 * pending, without hanging/breaking the guest.
 	 */
-	return __tdx_hypercall(&args, do_sti ? TDX_HCALL_ISSUE_STI : 0);
+	return __tdx_hypercall(&args, 0);
 }
 
 static int handle_halt(struct ve_info *ve)
 {
-	/*
-	 * Since non safe halt is mainly used in CPU offlining
-	 * and the guest will always stay in the halt state, don't
-	 * call the STI instruction (set do_sti as false).
-	 */
 	const bool irq_disabled = irqs_disabled();
-	const bool do_sti = false;
 
-	if (__halt(irq_disabled, do_sti))
+	if (__halt(irq_disabled))
 		return -EIO;
 
 	return ve_instr_len(ve);
@@ -210,22 +204,13 @@ static int handle_halt(struct ve_info *v
 
 void __cpuidle tdx_safe_halt(void)
 {
-	 /*
-	  * For do_sti=true case, __tdx_hypercall() function enables
-	  * interrupts using the STI instruction before the TDCALL. So
-	  * set irq_disabled as false.
-	  */
 	const bool irq_disabled = false;
-	const bool do_sti = true;
 
 	/*
 	 * Use WARN_ONCE() to report the failure.
 	 */
-	if (__halt(irq_disabled, do_sti))
+	if (__halt(irq_disabled))
 		WARN_ONCE(1, "HLT instruction emulation failed\n");
-
-	/* XXX I can't make sense of what @do_sti actually does */
-	raw_local_irq_disable();
 }
 
 static int read_msr(struct pt_regs *regs, struct ve_info *ve)
--- a/arch/x86/include/asm/shared/tdx.h
+++ b/arch/x86/include/asm/shared/tdx.h
@@ -8,7 +8,6 @@
 #define TDX_HYPERCALL_STANDARD  0
 
 #define TDX_HCALL_HAS_OUTPUT	BIT(0)
-#define TDX_HCALL_ISSUE_STI	BIT(1)
 
 #define TDX_CPUID_LEAF_ID	0x21
 #define TDX_IDENT		"IntelTDX    "