From: Daniel Borkmann daniel@iogearbox.net
commit ce02ef06fcf7a399a6276adb83f37373d10cbbe1 upstream.
From networking side, there are numerous attempts to get rid of indirect
calls in fast-path wherever feasible in order to avoid the cost of retpolines, for example, just to name a few:
* 283c16a2dfd3 ("indirect call wrappers: helpers to speed-up indirect calls of builtin") * aaa5d90b395a ("net: use indirect call wrappers at GRO network layer") * 028e0a476684 ("net: use indirect call wrappers at GRO transport layer") * 356da6d0cde3 ("dma-mapping: bypass indirect calls for dma-direct") * 09772d92cd5a ("bpf: avoid retpoline for lookup/update/delete calls on maps") * 10870dd89e95 ("netfilter: nf_tables: add direct calls for all builtin expressions") [...]
Recent work on XDP from Björn and Magnus additionally found that manually transforming the XDP return code switch statement with more than 5 cases into if-else combination would result in a considerable speedup in XDP layer due to avoidance of indirect calls in CONFIG_RETPOLINE enabled builds. On i40e driver with XDP prog attached, a 20-26% speedup has been observed [0]. Aside from XDP, there are many other places later in the networking stack's critical path with similar switch-case processing. Rather than fixing every XDP-enabled driver and locations in stack by hand, it would be good to instead raise the limit where gcc would emit expensive indirect calls from the switch under retpolines and stick with the default as-is in case of !retpoline configured kernels. This would also have the advantage that for archs where this is not necessary, we let compiler select the underlying target optimization for these constructs and avoid potential slow-downs by if-else hand-rewrite.
In case of gcc, this setting is controlled by case-values-threshold which has an architecture global default that selects 4 or 5 (latter if target does not have a case insn that compares the bounds) where some arch back ends like arm64 or s390 override it with their own target hooks, for example, in gcc commit db7a90aa0de5 ("S/390: Disable prediction of indirect branches") the threshold pretty much disables jump tables by limit of 20 under retpoline builds. Comparing gcc's and clang's default code generation on x86-64 under O2 level with retpoline build results in the following outcome for 5 switch cases:
* gcc with -mindirect-branch=thunk-inline -mindirect-branch-register:
# gdb -batch -ex 'disassemble dispatch' ./c-switch Dump of assembler code for function dispatch: 0x0000000000400be0 <+0>: cmp $0x4,%edi 0x0000000000400be3 <+3>: ja 0x400c35 <dispatch+85> 0x0000000000400be5 <+5>: lea 0x915f8(%rip),%rdx # 0x4921e4 0x0000000000400bec <+12>: mov %edi,%edi 0x0000000000400bee <+14>: movslq (%rdx,%rdi,4),%rax 0x0000000000400bf2 <+18>: add %rdx,%rax 0x0000000000400bf5 <+21>: callq 0x400c01 <dispatch+33> 0x0000000000400bfa <+26>: pause 0x0000000000400bfc <+28>: lfence 0x0000000000400bff <+31>: jmp 0x400bfa <dispatch+26> 0x0000000000400c01 <+33>: mov %rax,(%rsp) 0x0000000000400c05 <+37>: retq 0x0000000000400c06 <+38>: nopw %cs:0x0(%rax,%rax,1) 0x0000000000400c10 <+48>: jmpq 0x400c90 <fn_3> 0x0000000000400c15 <+53>: nopl (%rax) 0x0000000000400c18 <+56>: jmpq 0x400c70 <fn_2> 0x0000000000400c1d <+61>: nopl (%rax) 0x0000000000400c20 <+64>: jmpq 0x400c50 <fn_1> 0x0000000000400c25 <+69>: nopl (%rax) 0x0000000000400c28 <+72>: jmpq 0x400c40 <fn_0> 0x0000000000400c2d <+77>: nopl (%rax) 0x0000000000400c30 <+80>: jmpq 0x400cb0 <fn_4> 0x0000000000400c35 <+85>: push %rax 0x0000000000400c36 <+86>: callq 0x40dd80 <abort> End of assembler dump.
* clang with -mretpoline emitting search tree:
# gdb -batch -ex 'disassemble dispatch' ./c-switch Dump of assembler code for function dispatch: 0x0000000000400b30 <+0>: cmp $0x1,%edi 0x0000000000400b33 <+3>: jle 0x400b44 <dispatch+20> 0x0000000000400b35 <+5>: cmp $0x2,%edi 0x0000000000400b38 <+8>: je 0x400b4d <dispatch+29> 0x0000000000400b3a <+10>: cmp $0x3,%edi 0x0000000000400b3d <+13>: jne 0x400b52 <dispatch+34> 0x0000000000400b3f <+15>: jmpq 0x400c50 <fn_3> 0x0000000000400b44 <+20>: test %edi,%edi 0x0000000000400b46 <+22>: jne 0x400b5c <dispatch+44> 0x0000000000400b48 <+24>: jmpq 0x400c20 <fn_0> 0x0000000000400b4d <+29>: jmpq 0x400c40 <fn_2> 0x0000000000400b52 <+34>: cmp $0x4,%edi 0x0000000000400b55 <+37>: jne 0x400b66 <dispatch+54> 0x0000000000400b57 <+39>: jmpq 0x400c60 <fn_4> 0x0000000000400b5c <+44>: cmp $0x1,%edi 0x0000000000400b5f <+47>: jne 0x400b66 <dispatch+54> 0x0000000000400b61 <+49>: jmpq 0x400c30 <fn_1> 0x0000000000400b66 <+54>: push %rax 0x0000000000400b67 <+55>: callq 0x40dd20 <abort> End of assembler dump.
For sake of comparison, clang without -mretpoline:
# gdb -batch -ex 'disassemble dispatch' ./c-switch Dump of assembler code for function dispatch: 0x0000000000400b30 <+0>: cmp $0x4,%edi 0x0000000000400b33 <+3>: ja 0x400b57 <dispatch+39> 0x0000000000400b35 <+5>: mov %edi,%eax 0x0000000000400b37 <+7>: jmpq *0x492148(,%rax,8) 0x0000000000400b3e <+14>: jmpq 0x400bf0 <fn_0> 0x0000000000400b43 <+19>: jmpq 0x400c30 <fn_4> 0x0000000000400b48 <+24>: jmpq 0x400c10 <fn_2> 0x0000000000400b4d <+29>: jmpq 0x400c20 <fn_3> 0x0000000000400b52 <+34>: jmpq 0x400c00 <fn_1> 0x0000000000400b57 <+39>: push %rax 0x0000000000400b58 <+40>: callq 0x40dcf0 <abort> End of assembler dump.
Raising the cases to a high number (e.g. 100) will still result in similar code generation pattern with clang and gcc as above, in other words clang generally turns off jump table emission by having an extra expansion pass under retpoline build to turn indirectbr instructions from their IR into switch instructions as a built-in -mno-jump-table lowering of a switch (in this case, even if IR input already contained an indirect branch).
For gcc, adding --param=case-values-threshold=20 as in similar fashion as s390 in order to raise the limit for x86 retpoline enabled builds results in a small vmlinux size increase of only 0.13% (before=18,027,528 after=18,051,192). For clang this option is ignored due to i) not being needed as mentioned and ii) not having above cmdline parameter. Non-retpoline-enabled builds with gcc continue to use the default case-values-threshold setting, so nothing changes here.
[0] https://lore.kernel.org/netdev/20190129095754.9390-1-bjorn.topel@gmail.com/ and "The Path to DPDK Speeds for AF_XDP", LPC 2018, networking track: - http://vger.kernel.org/lpc_net2018_talks/lpc18_pres_af_xdp_perf-v3.pdf - http://vger.kernel.org/lpc_net2018_talks/lpc18_paper_af_xdp_perf-v2.pdf
Signed-off-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Thomas Gleixner tglx@linutronix.de Acked-by: Jesper Dangaard Brouer brouer@redhat.com Acked-by: Björn Töpel bjorn.topel@intel.com Acked-by: Linus Torvalds torvalds@linux-foundation.org Cc: netdev@vger.kernel.org Cc: David S. Miller davem@davemloft.net Cc: Magnus Karlsson magnus.karlsson@intel.com Cc: Alexei Starovoitov ast@kernel.org Cc: Peter Zijlstra peterz@infradead.org Cc: David Woodhouse dwmw2@infradead.org Cc: Andy Lutomirski luto@kernel.org Cc: Borislav Petkov bp@alien8.de Link: https://lkml.kernel.org/r/20190221221941.29358-1-daniel@iogearbox.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/Makefile | 5 +++++ 1 file changed, 5 insertions(+)
--- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -217,6 +217,11 @@ KBUILD_CFLAGS += -fno-asynchronous-unwin # Avoid indirect branches in kernel to deal with Spectre ifdef CONFIG_RETPOLINE KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) + # Additionally, avoid generating expensive indirect jumps which + # are subject to retpolines for small number of switch cases. + # clang turns off jump table generation by default when under + # retpoline builds, however, gcc does not for x86. + KBUILD_CFLAGS += $(call cc-option,--param=case-values-threshold=20) endif
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