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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Balbir Singh <bsingharora@gmail.com>,
	Madhavan Srinivasan <maddy@linux.vnet.ibm.com>,
	mpe@ellerman.id.au
Cc: linuxppc-dev@lists.ozlabs.org, paulus@samba.org, anton@samba.org,
	npiggin@gmail.com
Subject: Re: [PATCH v4 02/12] powerpc: move set_soft_enabled() and rename
Date: Tue, 20 Dec 2016 20:32:27 +1100	[thread overview]
Message-ID: <1482226347.15937.44.camel@kernel.crashing.org> (raw)
In-Reply-To: <f09a9213-9e85-eee0-e212-bd3ee2a3569e@gmail.com>

On Tue, 2016-12-20 at 20:03 +1100, Balbir Singh wrote:
> 
> On 19/12/16 19:06, Madhavan Srinivasan wrote:
> > Move set_soft_enabled() from powerpc/kernel/irq.c to
> > asm/hw_irq.c, to force updates to paca-soft_enabled
> > done via these access function. Add "memory" clobber
> > to hint compiler since paca->soft_enabled memory is the target
> > here
> > +static inline notrace void soft_enabled_set(unsigned long enable)
> > +{
> > +	__asm__ __volatile__("stb %0,%1(13)"
> > +	: : "r" (enable), "i" (offsetof(struct paca_struct,
> > soft_enabled))
> > +	: "memory");
> > +}
> > +
> 
> Can't we just rewrite this in "C"?
> 
> local_paca->soft_enabled = enable

Well, this is digging out another bloody can of baby eating worms...

So the reason we wrote it like that is because we had gcc playing
tricks to us.

It has been proved to move around references to local_paca, even
accross preempt_disable boundaries.

The above prevents it.

I'm about 100% sure we have a bunch of other issues related to PACA
access and gcc playing games though. I remember grepping the kernel
disassembly once for gcc doing funny things with r13 such as copying it
into another register or even saving and restoring it and my grep
didn't come up empty ...

Something we need to seriously look into one day. I have some ideas on
how to clean that up in the code to make hardening it easier, let's
talk next time Nick is in town.

Cheers,
Ben.

  reply	other threads:[~2016-12-20  9:32 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-19  8:06 [PATCH v4 00/12]powerpc: "paca->soft_enabled" based local atomic operation implementation Madhavan Srinivasan
2016-12-19  8:06 ` [PATCH v4 01/12] powerpc: Add #defs for paca->soft_enabled flags Madhavan Srinivasan
2016-12-20  7:31   ` Balbir Singh
2016-12-21  3:22     ` Madhavan Srinivasan
2016-12-19  8:06 ` [PATCH v4 02/12] powerpc: move set_soft_enabled() and rename Madhavan Srinivasan
2016-12-20  9:03   ` Balbir Singh
2016-12-20  9:32     ` Benjamin Herrenschmidt [this message]
2016-12-20 13:58       ` Balbir Singh
2016-12-21  3:32     ` Madhavan Srinivasan
2016-12-19  8:06 ` [PATCH v4 03/12] powerpc: Use soft_enabled_set api to update paca->soft_enabled Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 04/12] powerpc: Add soft_enabled manipulation functions Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 05/12] powerpc: reverse the soft_enable logic Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 06/12] powerpc: Avoid using EXCEPTION_PROLOG_1 macro in MASKABLE_* Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 07/12] powerpc: Add support to take additional parameter in MASKABLE_* macro Madhavan Srinivasan
2016-12-20  2:36   ` Nicholas Piggin
2016-12-21  6:14     ` Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 08/12] powerpc: Add support to mask perf interrupts and replay them Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 09/12] powerpc:Add new kconfig IRQ_DEBUG_SUPPORT Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 10/12] powerpc: Add new set of soft_enabled_ functions Madhavan Srinivasan
2016-12-20  2:42   ` Nicholas Piggin
2016-12-19  8:07 ` [PATCH v4 11/12] powerpc: rewrite local_t using soft_irq Madhavan Srinivasan
2016-12-19  8:07 ` [PATCH v4 12/12] powerpc: Rename soft_enabled to soft_disabled_mask Madhavan Srinivasan
2016-12-20  2:58   ` Nicholas Piggin
2017-01-04  9:33     ` Madhavan Srinivasan

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