Re: [PATCH v8 07/11] x86/tdx: Add HLT support for TDX guest

[Borislav Petkov]

On Mon, Oct 04, 2021 at 07:52:01PM -0700, Kuppuswamy Sathyanarayanan wrote:
> @@ -240,6 +243,33 @@ 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
> +	 * enables interrupts only one instruction later. If there
> +	 * are any instructions between the STI and the TDCALL for
> +	 * HLT then an interrupt could happen in that time, but the
> +	 * code would go back to sleep afterwards, which can cause
> +	 * longer delays.
> +	 *
> +	 * This leads to significant difference in network performance
> +	 * benchmarks. So add a special case for EXIT_REASON_HLT to
> +	 * trigger STI before TDCALL. But this change is not required
> +	 * for all HLT cases. So use R15 register value to identify the
> +	 * case which needs STI. So, if R11 is EXIT_REASON_HLT and R15
> +	 * is 1, then call STI before TDCALL instruction. Note that R15
> +	 * register is not required by TDCALL ABI when triggering the
> +	 * hypercall for EXIT_REASON_HLT case. So use it in software to
> +	 * select the STI case.
> +	 */
> +	cmpl $EXIT_REASON_HLT, %r11d
> +	jne skip_sti
> +	cmpl $1, %r15d
> +	jne skip_sti
> +	/* Set R15 register to 0, it is unused in EXIT_REASON_HLT case */
> +	xor %r15, %r15

Then you don't need to clear it either, right?

> +	sti
> +skip_sti:
>  	tdcall
>  
>  	/* Restore output pointer to R9 */

...

> +static __cpuidle void tdx_halt(void)
> +{
> +	const bool irq_disabled = irqs_disabled();
> +	const bool do_sti = false;
> +
> +	/*
> +	 * Non safe halt is mainly used in CPU off-lining
> +	 * and the guest will stay in halt state. So,
> +	 * STI instruction call is not required (set
> +	 * do_sti as false).
> +	 */

Just like you've done below, put the comment over the variable assignment:

        /*
         * Non safe halt is mainly used in CPU offlining  and the guest will stay in halt
         * state. So, STI instruction call is not required.
         */
        const bool do_sti = false;


> +	_tdx_halt(irq_disabled, do_sti);
> +}
> +
> +static __cpuidle void tdx_safe_halt(void)
> +{
> +	 /*
> +	  * Since STI instruction will be called in __tdx_hypercall()
> +	  * set irq_disabled as false.
> +	  */
> +	const bool irq_disabled = false;
> +	const bool do_sti = true;
> +
> +	_tdx_halt(irq_disabled, do_sti);
> +}
> +
>  unsigned long tdx_get_ve_info(struct ve_info *ve)
>  {
>  	struct tdx_module_output out = {0};

-- 
Regards/Gruss,
    Boris.

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