linux-kernel - Re: [PATCH V5 1/4] x86/paravirt: Move halt paravirt calls under CONFIG

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Message-ID: <CAGtprH-utPk=u938odDYAWAFLSZj+OhVH_k_RCHc87k3eqm0jQ@mail.gmail.com>
Date: Thu, 20 Feb 2025 15:17:10 -0800
From: Vishal Annapurve <vannapurve@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	virtualization@...ts.linux.dev, 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, chao.p.peng@...ux.intel.com, 
	isaku.yamahata@...il.com, sathyanarayanan.kuppuswamy@...ux.intel.com, 
	jgross@...e.com, ajay.kaher@...adcom.com, alexey.amakhalov@...adcom.com, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, stable@...r.kernel.org, 
	Andi Kleen <ak@...ux.intel.com>, Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH V5 1/4] x86/paravirt: Move halt paravirt calls under CONFIG_PARAVIRT

On Thu, Feb 20, 2025 at 1:47 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 2/20/25 13:16, Vishal Annapurve wrote:
> > Since enabling CONFIG_PARAVIRT_XXL is too bloated for TDX guest
> > like platforms, move HLT and SAFE_HLT paravirt calls under
> > CONFIG_PARAVIRT.
>
> I guess it's just one patch, but doesn't this expose CONFIG_PARAVIRT=y
> users to what _was_ specific to CONFIG_PARAVIRT_XXL=y? According to the
> changelog, TDX users shouldn't have to use use PARAVIRT_XXL, so
> PARAVIRT=y and PARAVIRT_XXL=n must be an *IMPORTANT* configuration for
> TDX users.
>
> Before this patch, those users would have no way to hit the
> unsafe-for-TDX pv_native_safe_halt(). After this patch, they will hit it.

Before this patch, those users had access to arch_safe_halt() ->
native_safe_halt() path. With this patch, such users can execute
arch_safe_halt -> pv_native_safe_halt() -> native_safe_halt(), so this
patch doesn't cause any additional regression.

>
> So, there are two possibilities:
>
>  1. This patch breaks bisection for an important TDX configuration
>  2. This patch's conjecture that PARAVIRT_XXL=n is important for TDX
>     is wrong and it is not necessary in the first place.
>
> What am I missing?