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

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Message-ID: <CAGtprH8mxENaH-Y0=b0kKJio=EG0OKt_qeguRBJECagXL4poPA@mail.gmail.com>
Date: Thu, 27 Feb 2025 11:26:53 -0800
From: Vishal Annapurve <vannapurve@...gle.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: dave.hansen@...ux.intel.com, kirill.shutemov@...ux.intel.com, 
	jgross@...e.com, ajay.kaher@...adcom.com, ak@...ux.intel.com, 
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	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 Wed, Feb 26, 2025 at 3:49 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> 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.
>

I will modify the description to include a paravirt spinlock example.

> >
> > 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