linux-kernel - Re: [RFC PATCH 01/12] powerpc/kernel: Get pt_regs from r9 before calling do_syscall_trace

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Date:	Fri, 17 Jul 2015 14:41:05 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linuxppc-dev@...abs.org, wad@...omium.org, keescook@...omium.org,
	linux-kernel@...r.kernel.org, luto@...capital.net,
	strosake@...ux.vnet.ibm.com, bogdan.purcareata@...escale.com
Subject: Re: [RFC PATCH 01/12] powerpc/kernel: Get pt_regs from r9 before
 calling do_syscall_trace_enter()

On Fri, 2015-07-17 at 08:40 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2015-07-15 at 17:37 +1000, Michael Ellerman wrote:
> > To call do_syscall_trace_enter() we need pt_regs in r3, but we don't need
> > to recalculate it based on r1, it's already in r9.
> > 
> > Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
> 
> Is there any performance difference ?

No.

I'm not going to bother measuring it :)

> I find the addi a bit more robust in case the code gets moved around or
> the "previous" code gets changed to either not use r9 or clobber it,
> which would have the potential to
> introduce a subtle bug ...

Yeah true.

There is an "invariant" in that entry code that r9 contains pt_regs, you can
see for example the DTL code goes to pains to ensure it puts pt_regs back in r9
after it clobbers it. As does the current syscall_dotrace.

But looking closer I don't see where we actually use that (prior to this
patch).

So yeah I'll drop this and send a clean up to just get rid of all the r9
reloading.

cheers



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