On Wed, Aug 27, 2025 at 10:47 AM Paolo Bonzini <pbonz...@redhat.com> wrote:
>
> On 8/27/25 10:26, Christian Ehrhardt wrote:
> > Hi,
> > in testing qemu 10.1 since rc levels I've found yet another odd
> > behavior. As far as I can tell it's not your fault, but I wanted to
> > tell you for awareness and potentially to help me get the right debug
> > data.
> >
> > A test that triggered when checking Ubuntu was the one of EDK2,
> > emulating riscv when running on ppc64, example log [1]. Yep, yet again
> > not the most common setup :-) and the same emulation in all other
> > Ubuntu architectures as host works fine.
> > Gladly this was reproducible and it eventually led me to a wild
> > journey which now makes me consider gcc-15 (15.2.0-1ubuntu1) as the
> > suspect here.
> >
> > Since I know the arch, the function and the tunable - I can use a
> > rather surgical mitigation like this.
> >
> > diff --git a/target/riscv/pmu.c b/target/riscv/pmu.c
> > index a68809eef3..5317d8be57 100644
> > --- a/target/riscv/pmu.c
> > +++ b/target/riscv/pmu.c
> > @@ -189,6 +189,13 @@ static int riscv_pmu_incr_ctr_rv64(RISCVCPU *cpu,
> > uint32_t ctr_idx)
> > * env->priv and env->virt_enabled contain old priv and old virt and
> > * new priv and new virt values are passed in as arguments.
> > */
> > +#if defined(__powerpc64__) || defined(__ppc64__)
> > + #define NO_GCSE_ATTR __attribute__((optimize("no-gcse")))
> > +#else
> > + #define NO_GCSE_ATTR
> > +#endif
> > +
> > +NO_GCSE_ATTR
> > static void riscv_pmu_icount_update_priv(CPURISCVState *env,
> > target_ulong newpriv, bool
> > new_virt)
> > {
> >
> > But a mitigation is all that it is, ideally, I'd report this as a gcc bug.
> > Yet the - understandable - hard requirement of getting the
> > pre-processed files makes this quite complex. As I can't even exactly
> > point to where exactly things go wrong.
> > I'd ask if one of you has experience in providing gcc-bugs out of a
> > qemu build. Is it as obvious as throwing -save-temps into *flags or is
> > there more to consider get what would be needed?
>
> Yes, it's like that. Run "ninja -v
> ./libqemu-riscv64-softmmu.a.p/target_riscv_pmu.c.o", stick -save-temps
> at the end and submit the resulting .i file as an attachment to the GCC bug.
Thanks for confirming, that allowed me to report it to gcc upstream as
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121689
Let us see what they say, and if anyone else is affected I hope you
can use the mitigation I shared until then.
> Paolo
>
--
Christian Ehrhardt
Director of Engineering, Ubuntu Server
Canonical Ltd
