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

["Huang, Kai" <kai.huang@intel.com>]

On Wed, 2025-06-04 at 12:46 -0700, Dave Hansen wrote:
> Circling back around now that I've divined out what is happening here...
> Is this even really about ENDBR at all? I thought that clang has some
> CFI checking that's completely independent from CET/IBT and thus ENDBR.
> Wouldn't this also break with that?

Hi Dave,

With your reminder on clang and CFI, I did more test on this patch, and
unfortunately I found building with clang and CONFIG_CFI_CLANG=y generates
below build error:

  ld.lld: error: undefined symbol: __kcfi_typeid___seamcall_saved_ret
  >>> referenced by usercopy_64.c
  >>>               vmlinux.o:(__cfi___seamcall_saved_ret)

It is because currently __seaamcall_saved_ret() is only called and is
directly called by tdh_vp_enter(), but not via sc_retry().

Making tdh_vp_enter() call __seamcall_saved_ret() via sc_retry() fixes
the error, but I am not sure it's a good idea since tdh_vp_enter() is in
performance critical path (although the additional cost of sc_retry() is
minor comparing to the SEAMCALL itself).

I think we can keep using SYM_FUNC_START for __seamcall_saved_ret() but only
change to using SYM_TYPED_FUNC_START for the other two.

I've sent out v2 based on above by replying to this v1 patch.  Appreciate if
you can review.  Thanks for your time.