linux-kernel - Re: [PATCH V3 1/2] x86/tdx: Route safe halt execution via tdx_safe

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Message-ID: <wra363f7ye6mwv2papahmpgmybi45yqyzeohunbqju3zsf22td@zcutpjluiury>
Date: Tue, 11 Feb 2025 10:32:40 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Vishal Annapurve <vannapurve@...gle.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com, 
	seanjc@...gle.com, erdemaktas@...gle.com, ackerleytng@...gle.com, jxgao@...gle.com, 
	sagis@...gle.com, oupton@...gle.com, pgonda@...gle.com, kirill@...temov.name, 
	dave.hansen@...ux.intel.com, linux-coco@...ts.linux.dev, chao.p.peng@...ux.intel.com, 
	isaku.yamahata@...il.com, stable@...r.kernel.org
Subject: Re: [PATCH V3 1/2] x86/tdx: Route safe halt execution via
 tdx_safe_halt()

On Thu, Feb 06, 2025 at 10:27:12PM +0000, 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. So "sti;hlt" sequence needs to
> be replaced with "TDCALL; raw_local_irq_enable()" for TDX VMs.
> 
> 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()
> 
> 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 changing state of interrupts.
> 
> If CONFIG_PARAVIRT_XXL is disabled, "sti;hlt" sequences can still get
> executed from TDX VMs via paths like:
>         acpi_safe_halt() =>
>         raw_safe_halt()  =>
>         arch_safe_halt() =>
> 	native_safe_halt()
> There is a long term plan to fix these paths by carving out
> irq.safe_halt() outside paravirt framework.

I don't think it is acceptable to keep !PARAVIRT_XXL (read no-Xen) config
broken.

We need either move irq.safe_halt() out of PARAVIRT_XXL now or make
non-paravirt arch_safe_halt() to use TDCALL. Or if we don't care about
performance of !PARAVIRT_XXL config, special-case HLT in
exc_virtualization_exception().

-- 
  Kiryl Shutsemau / Kirill A. Shutemov