linux-kernel - Re: [PATCH] drm/radeon: refactor CIK tiling table initialization

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Message-ID: <20160309174752.GF21308@treble.redhat.com>
Date:	Wed, 9 Mar 2016 11:47:52 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	"Deucher, Alexander" <Alexander.Deucher@....com>
Cc:	"Koenig, Christian" <Christian.Koenig@....com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kbuild test robot <fengguang.wu@...el.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] drm/radeon: refactor CIK tiling table initialization

On Mon, Mar 07, 2016 at 11:45:36PM +0000, Deucher, Alexander wrote:
> > -----Original Message-----
> > From: Josh Poimboeuf [mailto:jpoimboe@...hat.com]
> > Sent: Monday, March 07, 2016 6:10 PM
> > To: Deucher, Alexander; Koenig, Christian
> > Cc: dri-devel@...ts.freedesktop.org; linux-kernel@...r.kernel.org; kbuild
> > test robot; Ingo Molnar
> > Subject: [PATCH] drm/radeon: refactor CIK tiling table initialization
> > 
> > When compiling the radeon driver on x86_64 with
> > CONFIG_STACK_VALIDATION
> > enabled, objtool gives the following warnings:
> > 
> >   drivers/gpu/drm/radeon/cik.o: warning: objtool:
> > cik_tiling_mode_table_init()+0x6ce: call without frame pointer save/setup
> >   drivers/gpu/drm/radeon/cik.o: warning: objtool:
> > cik_tiling_mode_table_init()+0x72b: call without frame pointer save/setup
> >   drivers/gpu/drm/radeon/cik.o: warning: objtool:
> > cik_tiling_mode_table_init()+0x464: call without frame pointer save/setup
> >   ...
> > 
> > These are actually false positive warnings; there are no frame pointer
> > bugs.  Instead objtool gets confused by the jump tables created by all
> > the switch statements, combined with some other gcc optimizations.  It
> > tries to follows all possible code paths, but it fails to realize that
> > some of the paths aren't possible.  For example:
> > 
> >     4c97:       31 c0                   xor    %eax,%eax
> >     ...
> >     4ca2:       89 c1                   mov    %eax,%ecx
> >     4ca4:       ff 24 cd 00 00 00 00    jmpq   *0x0(,%rcx,8) 4ca7: R_X86_64_32S
> > .rodata+0x148
> > 
> > First eax is cleared to zero with the "xor %eax,%eax" instruction.
> > Later, it moves the value of eax (zero in this case) to ecx, and uses
> > that value to jump to the first entry in a jump table in .rodata.
> > 
> > Because objtool doesn't have an x86 emulator, it doesn't know that rcx
> > is zero.  So instead of following a single code path to the first jump
> > table entry, it follows all possible jump table entry paths in parallel.
> > 
> > Usually such overactive analysis isn't a problem.  In every other jump
> > table in the kernel, all the jump targets have the same frame pointer
> > state.  But in this exceedingly rare case, different targets have
> > different frame pointer states.  Objtool notices that and creates the
> > false positive warnings.
> > 
> > In theory we could use the STACK_FRAME_NON_STANDARD marker to tell
> > objtool to skip analysis of the function.  However, that's less than
> > ideal.
> > 
> > Looking at the cik_tiling_mode_table_init() code, it seems overly
> > complex with lots of repetition.  So let's simplify it.  All the switch
> > statements and conditionals can be replaced with much simpler logic by
> > generalizing the different behaviors and moving the initialization data
> > into data structures.
> > 
> > The change is a win-win: it's easier to parse for both humans and
> > machines.  It also reduces the binary size by about 2%:
> > 
> >      text	   data	    bss	    dec	    hex	filename
> >    101011	  30360	      0	 131371	  2012b	cik-before.o
> >     98699	  30200	      0	 128899	  1f783	cik-after.o
> > 
> > [ Note: Unfortunately I don't know how to test this code, so it's
> >   completely untested.  Any help or guidance with ensuring that the
> >   correct initialization is still being written would be greatly
> >   appreciated! ]
> 
> I think it would be clearer to rework it similarly to how it was
> reworked in amdgpu (see gfx_v8_0.c and gfx_v7_0.c in drm-next).  Also
> ideally you'd update the similar code in si.c as well for consistency.

Hi Alex,

Thanks for the pointers.  As it turns out, the false positive warning in
objtool was easier to fix than I originally thought, so this warning has
gone away.  But regardless I'll follow through and make a v2 patch based
on your suggestions.

-- 
Josh