Re: [PATCH V16 4/7] rust/powerpc: Set min rustc version for powerpc

[Mukesh Kumar Chaurasiya]

On Tue, May 26, 2026 at 02:22:49PM +0530, Mukesh Kumar Chaurasiya wrote:
> On Mon, May 25, 2026 at 08:16:53PM +0200, Miguel Ojeda wrote:
> > On Wed, May 20, 2026 at 8:48 AM Mukesh Kumar Chaurasiya (IBM)
> > <mkchauras@xxxxxxxxx> wrote:
> > >
> > > Minimum `rustc` version required for powerpc is 1.95 as some critical
> > > features required for compiling rust code for kernel are not there.
> > 
> > Which critical features?
> Hey Miguel,
> 
> Right now i can only think of inline asm. I can rerun the whole thing
> with 1.85 and figure out the issues with 1.85. I'll get back on this.
> 
> >
> > > For example Stable inline asm support which got merged in 1.95.
> > 
> > It is not needed that the support is stable, but rather that
> > everything you may need works.
> > 
> I wanted inline asm be stable, I was skeptical about inline asm to be
> unstable and potentially messing up the whole system. That's the reason
> I waited for the stable support to get merged before sending out this
> patch series.
> 
> > From a quick test (with a dummy example that may not be
> > representative), ppc64 inline assembly seems to work for a long time,
> > way before Rust 1.95.
> > 
> > So, which is the actual version that it is needed? i.e. 1.95.0 doesn't
> > seem to be required at least due to that.
> > 
> Yeah it may be true. I'll test out the 1.85 rustc and come back with the
> results.
> 
> > That is why I am asking about the critical features above, because it
> > may be that this works since earlier versions.
> > 
> > (I wonder if this patch and s390's similar one influenced each other?)
> >
> I am not aware of s390x way of approaching support for rust so can't say
> anything about that.
> 
> > Thanks!
> > 
> > Cheers,
> > Miguel
> 
> Regards,
> Mukesh
> 

Hey Miguel,

Sorry about delayed response on this.
I ran it with rustc 1.85.1.
CONFIG_RUSTC_VERSION_TEXT="rustc 1.85.1 (4eb161250 2025-03-15)"

I got a linker error with this.
ld.lld: error: undefined symbol: __subsf3
>>> referenced by core.7dc3da321ed68c1f-cgu.0
>>>               rust/core.o:(<core::core_arch::powerpc::altivec::vector_float as core::ops::arith::Neg>::neg) in archive vmlinux.a
>>> referenced by core.7dc3da321ed68c1f-cgu.0
>>>               rust/core.o:(<core::core_arch::powerpc::altivec::vector_float as core::ops::arith::Neg>::neg) in archive vmlinux.a
>>> referenced by core.7dc3da321ed68c1f-cgu.0
>>>               rust/core.o:(<core::core_arch::powerpc::altivec::vector_float as core::ops::arith::Neg>::neg) in archive vmlinux.a
>>> referenced 1 more times
make[2]: *** [scripts/Makefile.vmlinux:72: vmlinux.unstripped] Error 1
make[1]: *** [/home/mkchauras/src/linux/Makefile:1367: vmlinux] Error 2
make: *** [Makefile:248: __sub-make] Error 2

This corresponds to a floating point instruction. Although we have
disabled fpu, vsx and altivec we are still generating these. Seems like
we have some issue with the rustc here. I didn't dive deeper into this.

Is it ok if we keep 1.95 as base for powerpc64le?

Regards,
Mukesh