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

["Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 10/8/21 10:31 AM, Borislav Petkov wrote:
> 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?

Yes. As per ABI, I don't need to clear it. It will not be used by VMM.
But since that register content is shared with VMM, I tried to keep the
unused register values to '0' (consistent with other hypercall use cases).

I am fine with removing it if you think it is unnecessary.

> 
>> +	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;

Will move it above in next version.

> 
> 
>> +	_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};
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer