Re: [PATCH v3 14/19] x86/entry/64: Create a percpu SYSCALL entry trampoline

[Andy Lutomirski]

On Fri, Nov 24, 2017 at 3:39 AM, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Andy Lutomirski
>> Sent: 24 November 2017 04:33
> ...
>> +     /*
>> +      * x86 lacks a near absolute jump, and we can't jump to the real
>> +      * entry text with a relative jump, so we fake it using retq.
>> +      */
>> +     pushq   $entry_SYSCALL_64_after_hwframe
>> +     retq
>
> Don't some of the cpus have hardware logic that follows call and return?
> The above will break that logic and slow things down (unless it is an
> expected sequence).

Indeed, and it makes a *huge* difference.

>
> Is there a scratch register that can be used for an indirect jump?

No, but I can fudge it.

>
>         David
>