linux-kernel - Re: [PATCH] x86/virt/tdx: Add ENDBR for low level SEAMCALL assembly functions

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Message-ID: <ae0adccd-25d2-4dd3-b1b9-f7ba0c13367c@intel.com>
Date: Thu, 5 Jun 2025 07:54:48 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, Kai Huang <kai.huang@...el.com>
Cc: tglx@...utronix.de, bp@...en8.de, mingo@...hat.com, hpa@...or.com,
 kirill.shutemov@...ux.intel.com, rick.p.edgecombe@...el.com, x86@...nel.org,
 samitolvanen@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/virt/tdx: Add ENDBR for low level SEAMCALL assembly
 functions

On 6/5/25 07:21, Peter Zijlstra wrote:
> -static inline u64 sc_retry(sc_func_t func, u64 fn,
> -			   struct tdx_module_args *args)
> +static __always_inline u64 sc_retry(const sc_func_t func, u64 fn,
> +				    struct tdx_module_args *args)
>  {
>  	int retry = RDRAND_RETRY_LOOPS;

Practically, I can see how this works. If the compiler doesn't inline
sc_retry(), it stops being able to guarantee that the function pointer
value is known.

My only worry is that the compiler decides to be stupid for some other
reason, it could start generating indirect calls again.

Are we confident that __always_inline will keep the compiler from
generating indirect calls?