Re: [PATCH v3] Reduce assembly code size of exception entry points

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.02.2024 11:50, Frediano Ziglio wrote:
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -22,6 +22,14 @@
>  #endif
>  .endm
>  
> +.macro BUILD_BUG_ON condstr cond:vararg
> +        .if \cond
> +        .error "Condition \condstr not satisfied"

Maybe

        .error "Condition \"\condstr\" not satisfied"

?

> @@ -187,7 +195,8 @@ FUNC_LOCAL(restore_all_guest)
>          SPEC_CTRL_EXIT_TO_PV    /* Req: a=spec_ctrl %rsp=regs/cpuinfo, Clob: 
> cd */
>  
>          RESTORE_ALL
> -        testw $TRAP_syscall,4(%rsp)
> +        BUILD_BUG_ON(TRAP_syscall & 0xff)
> +        testb $TRAP_syscall >> 8,4+1(%rsp)
>          jz    iret_exit_to_guest

Nit: Blank after comma please (and again elsewhere).

Preferably with both adjustments (which I'd be okay making while
committing, so long as you agree specifically on the former)
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

That said, especially with this not being an entry point path, and
neither this nor ...

> @@ -254,7 +263,8 @@ FUNC(lstar_enter)
>          pushq $FLAT_KERNEL_CS64
>          pushq %rcx
>          pushq $0
> -        movl  $TRAP_syscall, 4(%rsp)
> +        BUILD_BUG_ON(TRAP_syscall & 0xff)
> +        movb  $TRAP_syscall >> 8, 4+1(%rsp)
>          SAVE_ALL
>  
>          SPEC_CTRL_ENTRY_FROM_PV /* Req: %rsp=regs/cpuinfo, %rdx=0, Clob: acd 
> */
> @@ -292,7 +302,8 @@ FUNC(cstar_enter)
>          pushq $FLAT_USER_CS32
>          pushq %rcx
>          pushq $0
> -        movl  $TRAP_syscall, 4(%rsp)
> +        BUILD_BUG_ON(TRAP_syscall & 0xff)
> +        movb  $TRAP_syscall >> 8, 4+1(%rsp)
>          SAVE_ALL
>  
>          SPEC_CTRL_ENTRY_FROM_PV /* Req: %rsp=regs/cpuinfo, %rdx=0, Clob: acd 
> */
> @@ -334,7 +345,8 @@ LABEL(sysenter_eflags_saved, 0)
>          pushq $3 /* ring 3 null cs */
>          pushq $0 /* null rip */
>          pushq $0
> -        movl  $TRAP_syscall, 4(%rsp)
> +        BUILD_BUG_ON(TRAP_syscall & 0xff)
> +        movb  $TRAP_syscall >> 8, 4+1(%rsp)
>          SAVE_ALL

... any of these being exception entry point paths (and hence none of
these changes being related to the subject), it would likely have been
a good idea to split this into two patches.

Jan