linux-kernel - Re: [PATCH V2 1/1] x86/tdx: Route safe halt execution via tdx_safe

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Message-ID: <40418980-6e5b-48eb-bc35-7ffaf3221fd9@intel.com>
Date: Mon, 3 Feb 2025 10:06:28 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Vishal Annapurve <vannapurve@...gle.com>,
 "Kirill A. Shutemov" <kirill@...temov.name>
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,
 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 V2 1/1] x86/tdx: Route safe halt execution via
 tdx_safe_halt()

On 1/31/25 18:32, Vishal Annapurve wrote:
...
> Are you hinting towards a model where TDX guest prohibits such call
> sites from being configured? I am not sure if it's a sustainable model
> if we just rely on the host not advertising these features as the
> guest kernel can still add new paths that are not controlled by the
> host that lead to *_safe_halt().

Let's say we required PARAVIRT_XXL for TDX guests and had TDX setup do:

static const typeof(pv_ops) tdx_irq_ops __initconst = {
        .irq = {
		.safe_halt = tdx_safe_halt,
	},
};

We could get rid of a _bit_ of what TDX is doing now, like:

        } else if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
                pr_info("using TDX aware idle routine\n");
                static_call_update(x86_idle, tdx_safe_halt);

and it would also fix this issue. Right?

This commit:

	bfe6ed0c6727 ("x86/tdx: Add HLT support for TDX guests")

Makes it seem totally possible:

>     Alternative choices like PV ops have been considered for adding
>     safe_halt() support. But it was rejected because HLT paravirt calls
>     only exist under PARAVIRT_XXL, and enabling it in TDX guest just for
>     safe_halt() use case is not worth the cost.

and honestly it's seeming more "worth the cost" now since that partial
approach has a bug and might have more bugs in the future.