[PATCH v3 22/51] x86/tdx: Remove TDX_HCALL_ISSUE_STI

Peter Zijlstra
2023-01-12T21:51:58Z

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@infradead.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Ulf Hansson <ulf.hansson@linaro.org>
---
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 "