Re: [PATCH 01/23] all: syscall wrappers: add documentation

[David Miller]

From: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
Date: Wed, 25 May 2016 23:03:27 +0300

> On Wed, May 25, 2016 at 12:30:17PM -0700, David Miller wrote:
>> From: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
>> Date: Tue, 24 May 2016 03:04:30 +0300
>> 
>> > +To clear that top halves, automatic wrappers are introduced. They clear all
>> > +required registers before passing control to regular syscall handler.
>> 
>> Why have one of these for every single compat system call, rather than
>> simply clearing the top half of all of these registers unconditionally
>> in the 32-bit system call trap before the system call is invoked?
>> 
>> That's what we do on sparc64.
>> 
>> And with that, you only need wrappers for the case where there needs
>> to be proper sign extention of a 32-bit signed argument.
> 
> It was discussed as one of possible solutions. The downside of it is
> that we cannot pass 64-bit types (like off_t) in single register.

Wrappers can be added for the cases where you'd like to do that.

> The other downside is that we clear top halves for every single
> syscall, and it looks excessive. So, from spark64 and s390 approaches
> we choosed second.

It's like 4 cpu cycles even on crappy sparc64 cpus which only dual
issue. :)

And that's a pretty low cost for the benefits if you ask me.