Re: [PATCH 2/4] x86: reduce CET-SS related #ifdef-ary

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jul 15, 2020 at 12:48:46PM +0200, Jan Beulich wrote:
> Commit b586a81b7a90 ("x86/CET: Fix build following c/s 43b98e7190") had
> to introduce a number of #ifdef-s to make the build work with older tool
> chains. Introduce an assembler macro covering for tool chains not
> knowing of CET-SS, allowing some conditionals where just SETSSBSY is the
> problem to be dropped again.
> 
> No change to generated code.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Looks like an improvement overall in code clarity:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Can you test on clang? Just to be on the safe side, otherwise I can
test it.

> ---
> Now that I've done this I'm not longer sure which direction is better to
> follow: On one hand this introduces dead code (even if just NOPs) into
> CET-SS-disabled builds. Otoh this is a step towards breaking the tool
> chain version dependency of the feature.
> 
> I've also dropped conditionals around bigger chunks of code; while I
> think that's preferable, I'm open to undo those parts.
> 
> --- a/xen/arch/x86/boot/x86_64.S
> +++ b/xen/arch/x86/boot/x86_64.S
> @@ -31,7 +31,6 @@ ENTRY(__high_start)
>          jz      .L_bsp
>  
>          /* APs.  Set up shadow stacks before entering C. */
> -#ifdef CONFIG_XEN_SHSTK
>          testl   $cpufeat_mask(X86_FEATURE_XEN_SHSTK), \
>                  CPUINFO_FEATURE_OFFSET(X86_FEATURE_XEN_SHSTK) + 
> boot_cpu_data(%rip)
>          je      .L_ap_shstk_done
> @@ -55,7 +54,6 @@ ENTRY(__high_start)
>          mov     $XEN_MINIMAL_CR4 | X86_CR4_CET, %ecx
>          mov     %rcx, %cr4
>          setssbsy
> -#endif
>  
>  .L_ap_shstk_done:
>          call    start_secondary
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -668,7 +668,7 @@ static void __init noreturn reinit_bsp_s
>      stack_base[0] = stack;
>      memguard_guard_stack(stack);
>  
> -    if ( IS_ENABLED(CONFIG_XEN_SHSTK) && cpu_has_xen_shstk )
> +    if ( cpu_has_xen_shstk )
>      {
>          wrmsrl(MSR_PL0_SSP,
>                 (unsigned long)stack + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 
> 8);
> --- a/xen/arch/x86/x86_64/compat/entry.S
> +++ b/xen/arch/x86/x86_64/compat/entry.S
> @@ -198,9 +198,7 @@ ENTRY(cr4_pv32_restore)
>  
>  /* See lstar_enter for entry register state. */
>  ENTRY(cstar_enter)
> -#ifdef CONFIG_XEN_SHSTK
>          ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
> -#endif
>          /* sti could live here when we don't switch page tables below. */
>          CR4_PV32_RESTORE
>          movq  8(%rsp),%rax /* Restore %rax. */
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -237,9 +237,7 @@ iret_exit_to_guest:
>   * %ss must be saved into the space left by the trampoline.
>   */
>  ENTRY(lstar_enter)
> -#ifdef CONFIG_XEN_SHSTK
>          ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK

Should the setssbsy be quoted, or it doesn't matter? I'm asking
because the same construction used by CLAC/STAC doesn't quote the
instruction.

Roger.