Re: [PATCH v5 08/12] x86/tdx: Add HLT support for TDX guest

Borislav Petkov
2021-08-24T16:10:15Z

On Wed, Aug 04, 2021 at 11:13:25AM -0700, Kuppuswamy Sathyanarayanan wrote:
> @@ -240,6 +243,32 @@ 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.

<-- newline in the comment here for better readability.

> There leads to significant difference in

"There leads..." ?

> + * 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,

s/sti/STI/g

> + * 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
> + sti
> +skip_sti:
> tdcall

...

> +static __cpuidle void tdg_safe_halt(void)
> +{
> + u64 ret;
> +
> + /*
> + * Enable interrupts next to the TDVMCALL to avoid
> + * performance degradation.

That comment needs some more love to say exactly what the problem is.

> + */
> + local_irq_enable();
> +
> + /* IRQ is enabled, So set R12 as 0 */
> + ret = _tdx_hypercall(EXIT_REASON_HLT, 0, 0, 0, 1, NULL);
> +
> + /* It should never fail */
> + BUG_ON(ret);
> +}

--
Regards/Gruss,
Boris.