Re: [PATCHv2.1 05/29] x86/tdx: Add HLT support for TDX guests

Thomas Gleixner
2022-02-01T21:22:03Z

On Sun, Jan 30 2022 at 01:30, Kirill A. Shutemov wrote:
> +/*
> + * Used in __tdx_hypercall() to determine whether to enable interrupts
> + * before issuing TDCALL for the EXIT_REASON_HLT case.
> + */
> +#define ENABLE_IRQS_BEFORE_HLT 0x01
> +
> /*
> * __tdx_module_call() - Used by TDX guests to request services from
> * the TDX module (does not include VMM services).
> @@ -230,6 +237,30 @@ 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.
> + *
> + * Since STI instruction is only required for the idle case
> + * (a special case of EXIT_REASON_HLT), use the r15 register
> + * value to identify it. Since the R15 register is not used
> + * by the VMM as per EXIT_REASON_HLT ABI, re-use it in
> + * software to identify the STI case.
> + */
> + cmpl $EXIT_REASON_HLT, %r11d
> + jne .Lskip_sti
> + cmpl $ENABLE_IRQS_BEFORE_HLT, %r15d
> + jne .Lskip_sti
> + /* Set R15 register to 0, it is unused in EXIT_REASON_HLT case */
> + xor %r15, %r15
> + sti
> +.Lskip_sti:
> tdcall

This really can be simplified:

cmpl $EXIT_REASON_SAFE_HLT, %r11d
jne .Lnohalt
movl $EXIT_REASON_HLT, %r11d
sti
.Lnohalt:
tdcall

and the below becomes:

static bool tdx_halt(void)
{
return !!__tdx_hypercall(EXIT_REASON_HLT, !!irqs_disabled(), 0, 0, 0, NULL);
}

void __cpuidle tdx_safe_halt(void)
{
if (__tdx_hypercall(EXIT_REASON_SAFE_HLT, 0, 0, 0, 0, NULL)
WARN_ONCE(1, "HLT instruction emulation failed\n");
}

Hmm?

Thanks,

tglx