Re: [PATCH V3 2/2] x86/tdx: Emit warning if IRQs are enabled during HLT #VE handling

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 06, 2025, Vishal Annapurve wrote:
> 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.
> 
> Emit warning and fail emulation if IRQs are enabled during HLT #VE handling
> to avoid running into scenarios where IRQ wake events are lost resulting in
> indefinite HLT execution times.
> 
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>@linux.intel.com>
> Signed-off-by: Vishal Annapurve <vannapurve@google.com>
> ---
>  arch/x86/coco/tdx/tdx.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 5e68758666a4..ed6738ea225c 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -393,6 +393,11 @@ static int handle_halt(struct ve_info *ve)
>  {
>  	const bool irq_disabled = irqs_disabled();
>  
> +	if (!irq_disabled) {

Wrap the check with WARN_ONCE(), doing so adds an unlikely to the <drum roll>
unlikely scenario.

> +		WARN_ONCE(1, "HLT instruction emulation unsafe with irqs enabled\n");

Newline is redundant, the WARN does that for you.  IMO, it's also worth adding
a comment, because this is like the fifth time "safe halt" has come up in the
context of TDX.

E.g.

	/*
	 * HLT with IRQs enabled is unsafe, as an IRQ that is intended to be a
	 * wake event may be consumed before requesting HLT emulation, leaving
	 * the vCPU blocking indefinitely.
	 */
	if (WARN_ONCE(!irq_disabled, "HLT emulation with IRQs enabled"))
		return -EIO;