linux-kernel - Re: [PATCH v6 2/3] x86/tdx: Fix arch_safe

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Message-ID: <pvbwlmkknw7cwln4onmi5mujpykyaxisb73khlriq7pzqhgno2@nvu3cbchp4am>
Date: Wed, 26 Feb 2025 13:49:31 +0200
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Vishal Annapurve <vannapurve@...gle.com>
Cc: dave.hansen@...ux.intel.com, kirill.shutemov@...ux.intel.com, 
	jgross@...e.com, ajay.kaher@...adcom.com, ak@...ux.intel.com, tony.luck@...el.com, 
	thomas.lendacky@....com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
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	chao.p.peng@...ux.intel.com, isaku.yamahata@...il.com, 
	sathyanarayanan.kuppuswamy@...ux.intel.com, erdemaktas@...gle.com, ackerleytng@...gle.com, jxgao@...gle.com, 
	sagis@...gle.com, afranji@...gle.com, kees@...nel.org, jikos@...nel.org, 
	peterz@...radead.org, x86@...nel.org, linux-kernel@...r.kernel.org, 
	linux-coco@...ts.linux.dev, virtualization@...ts.linux.dev, 
	bcm-kernel-feedback-list@...adcom.com, stable@...r.kernel.org
Subject: Re: [PATCH v6 2/3] x86/tdx: Fix arch_safe_halt() execution for TDX
 VMs

On Tue, Feb 25, 2025 at 12:47:03AM +0000, Vishal Annapurve wrote:
> Direct HLT instruction execution causes #VEs for TDX VMs which is routed
> to hypervisor via TDCALL. If HLT is executed in STI-shadow, resulting #VE
> handler will enable interrupts before TDCALL is routed to hypervisor
> leading to missed wakeup events.
> 
> Current TDX spec doesn't expose interruptibility state information to
> allow #VE handler to selectively enable interrupts. To bypass this
> issue, TDX VMs need to replace "sti;hlt" execution with direct TDCALL
> followed by explicit interrupt flag update.
> 
> Commit bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests")
> prevented the idle routines from executing HLT instruction in STI-shadow.
> 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()

I would rather use paravirt spinlock example. It is less controversial.
I still see no point in ACPI cpuidle be a thing in TDX guests.

> 
> To reliably handle arch_safe_halt() for TDX VMs, introduce explicit
> dependency on CONFIG_PARAVIRT and override paravirt halt()/safe_halt()
> routines with TDX-safe versions that execute direct TDCALL and needed
> interrupt flag updates. Executing direct TDCALL brings in additional
> benefit of avoiding HLT related #VEs altogether.
> 
> Cc: stable@...r.kernel.org
> Fixes: bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests")
> Signed-off-by: Vishal Annapurve <vannapurve@...gle.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov