Re: [Bpf] BPF ISA conformance groups

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Mon, Jan 8, 2024 at 8:00 AM Christoph Hellwig <hch@infradead.org> wrote:
>>
>> On Fri, Jan 05, 2024 at 04:07:11PM -0600, David Vernet wrote:
>> >
>> > So how do we want to move forward here? It sounds like we're leaning
>> > toward's Alexei's proposal of having:
>> >
>> > - Base Integer Instruction Set, 32-bit
>> > - Base Integer Instruction Set, 64-bit
>> > - Integer Multiplication and Division
>> > - Atomic Instructions
>>
>> As in the 64-bit integer set would be an add-on to the first one which
>> is the core set?  In that case that's fine with me, but the above
>> wording is a bit suboptimal.
>
> yes.
> Here is how I was thinking about the grouping:
> 32-bit set: all 32-bit instructions those with BPF_ALU and BPF_JMP32
> and load/store.
>
> 64-bit set: above plus BPF_ALU64 and BPF_JMP.
>
> The idea is to allow for clean 32-bit HW offloads.
> We can introduce a compiler flag that will only use such instructions
> and will error when 64-bit math is needed.
> Details need to be thought through, of course.
> Right now I'm not sure whether we need to reduce sizeof(void*) to 4
> in such a case or normal 8 will still work, but from ISA perspective
> everything is ready. 32-bit subregisters fit well.
> The compiler work plus additional verifier smartness is needed,
> but the end result should be very nice.
> Offload of bpf programs into 32-bit embedded devices will be possible.

This is very interesting.

Sounds like, on one hand, introducing ilp32 and lp64 C data models for
BPF:

ilp32

  int, long, pointers -> 32 bit
  short -> 16 bit
  char -> 8 bit

lp64

  long, pointers -> 64 bit
  int -> 32 bit
  short -> 16 bit
  char -> 8 bit

On the other hand, compiler flags -m32 and -m64 could determine what
instruction groups are generated and what C data model is used:

-m32

  Use ilp32 data model for C.
  Use 32-bit instruction set.

-m64

  Use lp64 data model for C.
  Use both 32-bit (if/when alu32) and 64-bit instruction sets.

And perhaps introducing a bit in the ELF flags section identifying a
32-bit binary.  Something like EF_BPF_32.

Would 64-bit ELF be used also in cases where BPF is offloaded to 32-bit
devices?

>> > And then either having 3 separate groups for the calls, or putting all 3
>> > in the basic group? I'd lean towards the latter given that we're
>> > decoupling ISA compliance from the verifier, but don't feel strongly
>> > either way.
>>
>> What would be the three different groups for the calls?  I think just
>> having the call instruction in the base group should be fine.  We'll
>> need to put in some wording that having support for any kind of call
>> depends on the program type.
>
> +1

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