Re: [PATCH v3] x86/virt/tdx: Enforce no indirect calls of TDX assembly

[Dave Hansen <dave.hansen@intel.com>]

It doesn't really "enforce" anything. But, oh well, I'll just fix it up
when I apply it early next week. Here's what I'll probably apply:

x86/virt/tdx: Avoid indirect calls to TDX assembly functions

Two 'static inline' TDX helper functions (sc_retry() and
sc_retry_prerr()) take function pointer arguments which refer to
assembly functions.  Normally, the compiler inlines the TDX helper,
realizes that the function pointer targets are completely static -- thus
can be resolved at compile time -- and generates direct call instructions.

But, other times (like when CONFIG_CC_OPTIMIZE_FOR_SIZE=y), the compiler
declines to inline the helpers and will instead generate indirect call
instructions.

Indirect calls to assembly functions require special annotation (for
various Control Flow Integrity mechanisms).  But TDX assembly functions
lack the special annotations and can only be called directly.

Annotate both the helpers as '__always_inline' to prod the compiler into
maintaining the direct calls. There is no guarantee here, but Peter has
volunteered to report the compiler bug if this assumption ever breaks[1].

...

> This was found through randconfig testing, presumably setting
> CONFIG_CC_OPTIMIZE_FOR_SIZE=1 when objtool spewed a bunch of these:
> 
> vmlinux.o: warning: objtool: tdh_mem_range_block+0x7e: relocation to
> !ENDBR: __seamcall_ret+0x0
> 
> Link: https://lore.kernel.org/lkml/20250605145914.GW39944@noisy.programming.kicks-ass.net/ [1]