Re: [Xen-devel] [PATCH v2 5/7] x86/traps: Lift all non-entrypoint logic in entry_int82() up into C

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.05.17 at 12:16, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 04/05/17 11:01, Andrew Cooper wrote:
> --- a/xen/arch/x86/x86_64/compat/entry.S
> +++ b/xen/arch/x86/x86_64/compat/entry.S
> @@ -21,6 +21,7 @@ ENTRY(entry_int82)
>  
>          mov   %rsp, %rdi
>          call  do_entry_int82
> +        jmp   compat_test_all_events
>  
>  /* %rbx: struct vcpu */
>  ENTRY(compat_test_all_events)
> 
> 
> This only currently works because the ALIGN hidden in ENTRY() happens to
> be safe to execute through, but we shouldn't rely on this,

Why not? ALIGN is the way it is for a reason.

> and a single predicted jump is probably faster to execute than
> a line of single-byte nops.

I'm not sure about this, but it's possible. A question is whether we
really should force 0x90 fillers in ALIGN, rather than having the
assembler use multi-bytes NOPs when suitable.

Jan


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