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

[Kuppuswamy, Sathyanarayanan]

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