From: Gabriel Paubert <paubert@iram.es>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Hari Bathini <hbathini@linux.vnet.ibm.com>,
linuxppc-dev <linuxppc-dev@ozlabs.org>,
Mahesh J Salgaonkar <mahesh@linux.vnet.ibm.com>,
Michael Neuling <mikey@neuling.org>,
Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH v3] ppc64/book3s: fix branching to out of line handlers in relocation kernel
Date: Fri, 1 Apr 2016 08:37:43 +0200 [thread overview]
Message-ID: <20160401063743.GA24955@visitor2.iram.es> (raw)
In-Reply-To: <1459491275.10334.6.camel@ellerman.id.au>
Hi Michael,
On Fri, Apr 01, 2016 at 05:14:35PM +1100, Michael Ellerman wrote:
> On Wed, 2016-03-30 at 23:49 +0530, Hari Bathini wrote:
> > Some of the interrupt vectors on 64-bit POWER server processors are
> > only 32 bytes long (8 instructions), which is not enough for the full
> ...
> > Let us fix this undependable code path by moving these OOL handlers below
> > __end_interrupts marker to make sure we also copy these handlers to real
> > address 0x100 when running a relocatable kernel. Because the interrupt
> > vectors branching to these OOL handlers are not long enough to use
> > LOAD_HANDLER() for branching as discussed above.
> >
> ...
> > changes from v2:
> > 2. Move the OOL handlers before __end_interrupts marker instead of moving the __end_interrupts marker
> > 3. Leave __end_handlers marker as is.
>
> Hi Hari,
>
> Thanks for trying this. In the end I've decided it's not a good option.
>
> If you build an allmodconfig, and turn on CONFIG_RELOCATABLE, and then look at
> the disassembly, you see this:
>
> c000000000006ffc: 48 00 29 04 b c000000000009900 <.ret_from_except>
>
> c000000000007000 <__end_handlers>:
>
> At 0x7000 we have the FWNMI area, which is fixed and can't move. As you see
> above we end up with only 4 bytes of space between the end of the handlers and
> the FWNMI area.
Nitpicking a bit, if I correctly read the above disassembly and there is an instuction
at 0x6ffc, the free space is exactly 0!
>
> So any tiny change that adds two more instructions prior to 0x7000 will then
> fail to build.
Even one instruction provided I still know how to count.
>
> None of that's your fault, it's just the nature of the code in there, it's very
> space constrained.
Calling it space very constrained makes you win the understatement of the month
award, on April fool's day :-)
Regards,
Gabriel
next prev parent reply other threads:[~2016-04-01 8:10 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-30 18:19 [PATCH v3] ppc64/book3s: fix branching to out of line handlers in relocation kernel Hari Bathini
2016-04-01 6:14 ` Michael Ellerman
2016-04-01 6:37 ` Gabriel Paubert [this message]
2016-04-01 10:40 ` Michael Ellerman
2016-04-01 6:53 ` Hari Bathini
2016-04-01 10:37 ` Michael Ellerman
2016-04-01 19:41 ` Hari Bathini
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