[llvm-dev] [iovisor-dev] [PATCH RFC 0/4] Initial 32-bit eBPF encoding support

[Y Song via llvm-dev]

On Sat, Sep 23, 2017 at 1:41 AM, Jakub Kicinski via iovisor-dev
<iovisor-dev at lists.iovisor.org> wrote:
> On Fri, 22 Sep 2017 22:03:47 -0700, Yonghong Song wrote:
>> On 9/22/17 9:24 AM, Jakub Kicinski wrote:
>> > On Thu, 21 Sep 2017 11:56:55 -0700, Alexei Starovoitov wrote:
>> >> On Wed, Sep 20, 2017 at 12:20:40AM +0100, Jiong Wang via iovisor-dev wrote:
>> >>> On 18/09/2017 22:29, Daniel Borkmann wrote:
>> >>>> On 09/18/2017 10:47 PM, Jiong Wang wrote:
>> >>>>> Hi,
>> >>>>>
>> >>>>>     Currently, LLVM eBPF backend always generate code in 64-bit mode,
>> >>>>> this may
>> >>>>> cause troubles when JITing to 32-bit targets.
>> >>>>>
>> >>>>>     For example, it is quite common for XDP eBPF program to access
>> >>>>> some packet
>> >>>>> fields through base + offset that the default eBPF will generate
>> >>>>> BPF_ALU64 for
>> >>>>> the address formation, later when JITing to 32-bit hardware,
>> >>>>> BPF_ALU64 needs
>> >>>>> to be expanded into 32 bit ALU sequences even though the address
>> >>>>> space is
>> >>>>> 32-bit that the high bits is not significant.
>> >>>>>
>> >>>>>     While a complete 32-bit mode implemention may need an new ABI
>> >>>>> (something like
>> >>>>> -target-abi=ilp32), this patch set first add some initial code so we
>> >>>>> could
>> >>>>> construct 32-bit eBPF tests through hand-written assembly.
>> >>>>>
>> >>>>>     A new 32-bit register set is introduced, its name is with "w"
>> >>>>> prefix and LLVM
>> >>>>> assembler will encode statements like "w1 += w2" into the following
>> >>>>> 8-bit code
>> >>>>> field:
>> >>>>>
>> >>>>>       BPF_ADD | BPF_X | BPF_ALU
>> >>>>>
>> >>>>> BPF_ALU will be used instead of BPF_ALU64.
>> >>>>>
>> >>>>>     NOTE, currently you can only use "w" register with ALU
>> >>>>> statements, not with
>> >>>>> others like branches etc as they don't have different encoding for
>> >>>>> 32-bit
>> >>>>> target.
>> >>>>
>> >>>> Great to see work in this direction! Can we also enable to use / emit
>> >>>> all the 32bit BPF_ALU instructions whenever possible for the currently
>> >>>> available bpf targets while at it (which only use BPF_ALU64 right now)?
>> >>>
>> >>> Hi Daniel,
>> >>>
>> >>>     Thanks for the feedback.
>> >>>
>> >>>     I think we could also enable the use of all the 32bit BPF_ALU under
>> >>> currently
>> >>> available bpf targets.  As we now have 32bit register set support, we could
>> >>> make
>> >>> i32 type as legal type to prevent it be promoted into i64, then hook it up
>> >>> with i32
>> >>> ALU patterns, will look into this.
>> >>
>> >> I don't think we need to gate 32bit alu generation with a flag.
>> >> Though interpreter and JITs support 32-bit since day one, the verifier
>> >> never seen such programs before, so some valid programs may get
>> >> rejected. After some time passes and we're sure that all progs
>> >> still work fine when they're optimized with 32-bit alu, we can flip
>> >> the switch in llvm and make it default.
>> >
>> > Thinking about next steps - do we expect the 32b operations to clear the
>> > upper halves of the registers?  The interpreter does it, and so does
>> > x86.  I don't think we can load 32bit-only programs on 64bit hosts, so
>> > we would need some form of data flow analysis in the kernel to prune
>> > the zeroing for 32bit offload targets.  Is that correct?
>>
>> Could you contrive an example to show the problem? If I understand
>> correctly, you most worried that some natural sign extension is gone
>> with "clearing the upper 32-bit register" and such clearing may make
>> some operation, esp. memory operation not correct in 64-bit machine?
>
> Hm.  Perhaps it's a blunder on my side, but let's take:
>
>   r1 = ~0ULL
>   w1 = 0
>   # use r1
>
> on x86 and the interpreter, the w1 = 0 will clear upper 32bits, so r1
> ends up as 0.  32b arches may translate this to something like:
>
>   # r1 = ~0ULL
>   r1.lo = ~0
>   r1.hi = ~0
>   # w1 = 0
>   r1.lo = 0
>   # r1.hi not touched
>
> which will obviously result in r1 == 0xffffffff00000000.  LLVM should
> not assume r1.hi is cleared, but I'm not sure this is a strong enough
> argument.

Not sure what LLVM will do in this case  for later "r1" access
unless going through the real implementation. My hunch is LLVM
should do a conversion from 32bit to 64bit, "r1 <<= 32" and
"r1 >>= 32" after "w1 = 0" before using r1. Let us wait and check
once implementation in place.

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