linux-kernel - Re: [RFC/INCOMPLETE 08/13] x86/entry/64: Migrate 64-bit syscalls to new exit hooks

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Date:	Thu, 18 Jun 2015 12:19:43 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC/INCOMPLETE 08/13] x86/entry/64: Migrate 64-bit syscalls to
 new exit hooks


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Andy Lutomirski <luto@...capital.net> wrote:
> 
> > >> Any reason why irq state tracking cannot be done in C as well, like the 
> > >> rest of the irq state tracking code?
> > >
> > > Never mind, I see you've done exactly that in patch #12.
> > 
> > There are still some TRACE_IRQS_ON, LOCKDEP_SYS_EXIT, and such scattered 
> > throughout the asm.  it's plausible that even more of that could be moved to 
> > C.
> > 
> > We could also benchmark and find out how bad it would be if we just always 
> > filled pt_regs in completely in syscalls.  If the performance hit isn't enough 
> > to matter, then we could potentially move the entire syscall path except 
> > pt_regs setup and sysret/iret into three C functions.
> 
> The thing is, I'd not be against simplifying pt_regs handling even if it slows 
> down things a tiny bit. If anyone wants to reintroduce that complexity we'll see 
> how it looks like in isolation, done cleanly.

... and I suspect the reduction of entry points will allow the compiler to do a 
better job - so some of the overhead might be won back.

So I'd say we try this approach and complicate it back in the future only if the 
numbers warrant it.

Thanks,

	Ingo
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