Re: [PATCH v7 08/12] x86/tdx: Add HLT support for TDX guest

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

On 9/23/21 11:09 AM, Borislav Petkov wrote:
> On Thu, Sep 16, 2021 at 11:35:46AM -0700, Kuppuswamy Sathyanarayanan wrote:
>> +static __cpuidle void _tdx_halt(const bool irq_disabled, const bool do_sti)
>> +{
>> +	u64 ret;
>> +
>> +	/*
>> +	 * Emulate HLT operation via hypercall. More info about ABI
>> +	 * can be found in TDX Guest-Host-Communication Interface
>> +	 * (GHCI), sec 3.8.
> 
> 	"3.8 TDG.VP.VMCALL<Instruction.HLT>"
> 
> write that section name because those numbers do change.

Sorry, I fixed it in commit log, but missed it here. I will fix it in next
submission.

> 
>> +	 *
>> +	 * The VMM uses the "IRQ disabled" param to understand IRQ
>> +	 * enabled status (RFLAGS.IF) of TD guest and determine
>> +	 * whether or not it should schedule the halted vCPU if an
>> +	 * IRQ becomes pending. E.g. if IRQs are disabled the VMM
>> +	 * can keep the vCPU in virtual HLT, even if an IRQ is
>> +	 * pending, without hanging/breaking the guest.
>> +	 *
>> +	 * do_sti parameter is used by __tdx_hypercall() to decide
>> +	 * whether to call STI instruction before executing TDCALL
>> +	 * instruction.
>> +	 */
>> +	ret = _tdx_hypercall(EXIT_REASON_HLT, irq_disabled, 0, 0, do_sti, NULL);
> 
> So that irq_disabled goes into r12. Nothing in that section 3.8 above
> talks about r12. The doc version I'm looking at is:
> 
> 344426-001US
> SEPTEMBER 2020
> 
> Where is that "the IRQs in the guest were disabled/enabled" bit
> documented?

IRQ parameter specification update is not yet released for public. I think it will
be released in 2-3 weeks.

> 
>> +
>> +	/*
>> +	 * Use WARN_ONCE() to report the failure. Since tdx_*halt() calls
>> +	 * are also used in pv_ops, #VE handler error handler cannot be
> 
> one "handler"'s enough.

Ok. I will fix this in next version.

> 
>> +	 * used to report the failure.
>> +	 */
>> +	WARN_ONCE(ret, "HLT instruction emulation failed\n");
>> +}
>> +
>> +static __cpuidle void tdx_halt(void)
>> +{
>> +	const bool irq_disabled = irqs_disabled();
>> +	const bool do_sti = false;
> 
> What is the logic here?
> 
> This is not a safe halt so it doesn't matter to the TDX module whether
> irqs are disabled or not?
> 
> That comment above is again keeping it to itself:
> 
> "But this change is not required for all HLT cases."
> 
> So for which cases is it required?

It's only needed for the safe hlt case because the non safe hlt case doesn't
change anything about the interrupt.

> 
> Is that explained in the comment in _tdx_halt() where irqs_disabled
> tells the VMM what to do with the guest - to wake it up or to keep it in
> virtual halt?

I think it is left in halt state. Sean, any comment?

> 
>> +
>> +	_tdx_halt(irq_disabled, do_sti);
>> +}
>> +
>> +static __cpuidle void tdx_safe_halt(void)
>> +{
>> +	const bool irq_disabled = false; /* since sti will be called */
> 
> Comments usually go ontop not on the side.

I will fix this in next version.

> 
>> +	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};
> 
> Thx.
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer