RE: [Bpf] BPF ISA conformance groups

[dthaler1968@googlemail.com]

> -----Original Message-----
> From: David Vernet <void@manifault.com>
> Sent: Tuesday, January 23, 2024 1:40 PM
> To: Jose E. Marchesi <jose.marchesi@oracle.com>
> Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>; Christoph Hellwig
> <hch@infradead.org>; Dave Thaler <dthaler1968@googlemail.com>;
> bpf@ietf.org; bpf <bpf@vger.kernel.org>; Jakub Kicinski <kuba@kernel.org>;
> david.faust@oracle.com
> Subject: Re: [Bpf] BPF ISA conformance groups
> 
> On Tue, Jan 09, 2024 at 12:35:39PM +0100, Jose E. Marchesi wrote:
> >
> > > 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.
> this is necessarily something we need to figure out now. Hopefully this is all
> stuff we can iron out once we start to really sink our teeth into the ABI doc.

"Integer Multiplication and Division" in this thread doesn't seem to separate
between 32-bit vs 64-bit.  Is the proposal that multiplication/division is ok
to require 64-bit operations?  I had expected one rationale for the 32bit
multiplication/division instructions is to accommodate 32-bit-only
implementations.   So should we have separate groups for 32-bit vs
64-bit for the multiplication/division instructions?

Similar question goes for the atomic instructions, i.e., should we
have separate conformance groups for 32-bit vs 64-bit atomics? 

Dave