On Thu, Feb 20, 2025 at 3:00 PM Dave Hansen dave.hansen@intel.com wrote:
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.
pv_native_safe_halt() -> native_safe_halt() -> "sti; hlt".
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?
Makes sense, I will update the commit message in the next version to clearly answer these questions and address the above comments.