Re: [PATCH V5 2/4] x86/tdx: Route safe halt execution via tdx_safe_halt()

[Dave Hansen <dave.hansen@intel.com>]

On 2/20/25 13:16, Vishal Annapurve wrote:
> Direct HLT instruction execution causes #VEs for TDX VMs which is routed
> to hypervisor via TDCALL. safe_halt() routines execute HLT in STI-shadow
> so IRQs need to remain disabled until the TDCALL to ensure that pending
> IRQs are correctly treated as wake events.

This isn't quite true. There's only one paravirt safe_halt() and it
doesn't do HLT or STI.

I think it's more true to say that "safe" halts are entered with IRQs
disabled. They logically do the halt operation and then enable
interrupts before returning.

> So "sti;hlt" sequence needs to be replaced for TDX VMs with "TDCALL;
> *_irq_enable()" to keep interrupts disabled during TDCALL execution.
But this isn't new. TDX already tried to avoid "sti;hlt". It just
screwed up the implementation.

> Commit bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests")
> prevented the idle routines from using "sti;hlt". But it missed the
> paravirt routine which can be reached like this as an example:
>         acpi_safe_halt() =>
>         raw_safe_halt()  =>
>         arch_safe_halt() =>
>         irq.safe_halt()  =>
>         pv_native_safe_halt()

This, on the other hand, *is* important.

> Modify tdx_safe_halt() to implement the sequence "TDCALL;
> raw_local_irq_enable()" and invoke tdx_halt() from idle routine which just
> executes TDCALL without toggling interrupt state. Introduce dependency
> on CONFIG_PARAVIRT and override paravirt halt()/safe_halt() routines for
> TDX VMs.

This changelog glosses over one of the key points: Why *MUST* TDX use
paravirt? It further confuses the reasoning by alluding to the idea that
"Direct HLT instruction execution ... is routed to hypervisor via TDCALL".

It gives background and a solution, but it's not obvious what the
problem is or how the solution _fixes_ the problem.

What must TDX now depend on PARAVIRT?

Why not just route the HLT to a TDXCALL via the #VE code?