Subject: Re: [PATCH net-next] fast_hash: clobber registers correctly for inline function use

From: Hannes Frederic Sowa <[email-masked]>
Date: Fri, 14 Nov 2014 23:37:42 +0100

Hi Jay,

On Fr, 2014-11-14 at 14:10 -0800, Jay Vosburgh wrote:
> Hannes Frederic Sowa <[email-masked]> wrote:
> [...]
> >I created it via the function calling convention documented in
> >arch/x86/include/asm/calling.h, so I specified each register which a
> >function is allowed to clobber with.
> >
> >I currently cannot see how I can resolve the invalid constraints error
> >easily. :(
> >
> >So either go with my first patch, which I puts the alternative_call
> >switch point into its own function without ever inlining or the patch
> >needs to be reverted. :/
> 
> 	As a data point, I tested the first patch as well, and the
> system does not panic with it in place.  Inspection shows that it's
> using %r14 in place of %r8 in the prior (crashing) implementation.

Yes, I also could reproduce your oops and the first unoffical patch  and
the first offical one both fixed it. After that, I thought that just
adding more clobbers cannot introduce bugs, so I only did compile
testing until I hit a window where gcc got mad with the excessive use of
clobbered registers but haven't tested the inline call sites that much
(sorry). :(

> 	Disassembly of the call site (on the non-sse4_1 system) in
> ovs_flow_tbl_insert with the first patch applied looks like this:
> 
> 0xffffffffa00b6bb9 <ovs_flow_tbl_insert+0xb9>:	mov    %r15,0x348(%r14)
> 0xffffffffa00b6bc0 <ovs_flow_tbl_insert+0xc0>:	movzwl 0x28(%r15),%ecx
> 0xffffffffa00b6bc5 <ovs_flow_tbl_insert+0xc5>:	movzwl 0x2a(%r15),%esi
> 0xffffffffa00b6bca <ovs_flow_tbl_insert+0xca>:	movzwl %cx,%eax
> 0xffffffffa00b6bcd <ovs_flow_tbl_insert+0xcd>:	sub    %ecx,%esi
> 0xffffffffa00b6bcf <ovs_flow_tbl_insert+0xcf>:	lea    0x38(%r14,%rax,1),%rdi
> 0xffffffffa00b6bd4 <ovs_flow_tbl_insert+0xd4>:	sar    $0x2,%esi
> 0xffffffffa00b6bd7 <ovs_flow_tbl_insert+0xd7>:	callq  0xffffffff813a7810 <__jhash2>
> 0xffffffffa00b6bdc <ovs_flow_tbl_insert+0xdc>:	mov    %eax,0x30(%r14)
> 0xffffffffa00b6be0 <ovs_flow_tbl_insert+0xe0>:	mov    (%rbx),%r13
> 0xffffffffa00b6be3 <ovs_flow_tbl_insert+0xe3>:	mov    %r14,%rsi
> 0xffffffffa00b6be6 <ovs_flow_tbl_insert+0xe6>:	mov    %r13,%rdi
> 0xffffffffa00b6be9 <ovs_flow_tbl_insert+0xe9>:	callq  0xffffffffa00b61a0 <table_instance_insert>
> 
> 	Compared to the panicking version's function:
> 
> 0xffffffffa01a55c9 <ovs_flow_tbl_insert+0xb9>:  mov    %r15,0x348(%r8)
> 0xffffffffa01a55d0 <ovs_flow_tbl_insert+0xc0>:  movzwl 0x28(%r15),%ecx
> 0xffffffffa01a55d5 <ovs_flow_tbl_insert+0xc5>:  movzwl 0x2a(%r15),%esi
> 0xffffffffa01a55da <ovs_flow_tbl_insert+0xca>:  movzwl %cx,%eax
> 0xffffffffa01a55dd <ovs_flow_tbl_insert+0xcd>:  sub    %ecx,%esi
> 0xffffffffa01a55df <ovs_flow_tbl_insert+0xcf>:  lea    0x38(%r8,%rax,1),%rdi
> 0xffffffffa01a55e4 <ovs_flow_tbl_insert+0xd4>:  sar    $0x2,%esi
> 0xffffffffa01a55e7 <ovs_flow_tbl_insert+0xd7>:  callq  0xffffffff813a75c0 <__jhash2>
> 0xffffffffa01a55ec <ovs_flow_tbl_insert+0xdc>:  mov    %eax,0x30(%r8)
> 0xffffffffa01a55f0 <ovs_flow_tbl_insert+0xe0>:  mov    (%rbx),%r13
> 0xffffffffa01a55f3 <ovs_flow_tbl_insert+0xe3>:  mov    %r8,%rsi
> 0xffffffffa01a55f6 <ovs_flow_tbl_insert+0xe6>:  mov    %r13,%rdi
> 0xffffffffa01a55f9 <ovs_flow_tbl_insert+0xe9>:  callq  0xffffffffa01a4ba0 <table_instance_insert>
> 
> 	It appears to generate the same instructions, but allocates
> registers differently (using %r14 instead of %r8).

Exactly and that makes sense. While %r8 must be available for the callee
to be clobbered with, %r14 must be saved by the callee and restored
before returning. So you pass the responsibility down to the other
functions, which tries not to touch %r14 because it knows it will have
to generate code for saving and restoring.

That's the reason why I actually like the the static inline clobbering
approach so much, it gives gcc possibilities to move around the
save/restore cycles and decide itself just by aligning which registers
to use.

Also the first version does work flawlessly (which I didn't send as a
patch but only as a diff in the mail). Here gcc synthesizes a full
function call which has the same effect as the long clobber list, only
it does chain two calls right behind each other.

> 	The __jhash2 disassembly appears to be unchanged between the two
> versions.

Thanks for looking into this!

It is actually pretty hairy to come up with a good solution for this,
because with the alternative interface you are only allowed to alter one
instruction. jump_tables also don't work because I currently have the
opinion that they do the switch way too late. I absolutely don't want to
have inserts into a hashtable with different hashing functions depending
how early during boot they took place.

Bye,
Hannes