Re: [PATCH v7 1/6] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

On Fri, Feb 09, 2024 at 08:57:04PM +0100, Borislav Petkov wrote:
> On Fri, Feb 09, 2024 at 11:06:02AM -0800, Pawan Gupta wrote:
> > (Though, there was a comment on avoiding the macro alltogether, to which
> > I replied that it complicates 32-bit.)
> 
> Hmm, so this seems to build the respective entry_{32,64}.S TUs fine:
> 
> ---
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index e81cabcb758f..7d1e5fe66495 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -313,12 +313,9 @@
>   *
>   * Note: Only the memory operand variant of VERW clears the CPU buffers.
>   */
> -.macro EXEC_VERW
> -	verw _ASM_RIP(mds_verw_sel)
> -.endm
>  
>  .macro CLEAR_CPU_BUFFERS
> -	ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF
> +	ALTERNATIVE "",  __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
>  .endm

> +.macro CLEAR_CPU_BUFFERS
> +     ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF
> +.endm

This works, thanks.

My mistake that I tried your earlier suggestion without playing with it:

  > > +.macro CLEAR_CPU_BUFFERS
  > > +   ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF
  > > +.endm
  >
  > Why can't this simply be:
  >
  > .macro CLEAR_CPU_BUFFERS
  >         ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
  > .endm
  >
  > without that silly EXEC_VERW macro?
  >
  > --
  > Regards/Gruss,
  >     Boris.