Re: [PATCH 4/7] spapr: Fix record-replay machine reset consuming too many events

["Nicholas Piggin" <npiggin@gmail.com>]

On Mon Jun 26, 2023 at 6:07 PM AEST, Pavel Dovgalyuk wrote:
> e500 has the same problem, I think, according to this issue: 
> https://gitlab.com/qemu-project/qemu/-/issues/1634

Same symptoms. e500 looks like it does the dt build in
machine_init_done notifier, though. Maybe I miss something.
I'll take a look.

>
> Btw, ARM virt platform rebuilds fdt only at initialization phase, not 
> when reset.

I was actually wondering about keeping the same rng-seed across resets
to make the code even simpler, but decided to keep behaviour unchanged.
That seems like one downside.

> Isn't this behavior correct? Shouldn't PPC platforms do the similar thing?

I believe spapr does this for an spapr "feature" that rebuilds the fdt
at runtime ("client-architecture-support"), so reset has to build the
original one again.

Thanks,
Nick

>
> On 23.06.2023 15:57, Nicholas Piggin wrote:
> > spapr_machine_reset gets a random number to populate the device-tree
> > rng seed with. When loading a snapshot for record-replay, the machine
> > is reset again, and that tries to consume the random event record
> > again, crashing due to inconsistent record
> > 
> > Fix this by saving the seed to populate the device tree with, and
> > skipping the rng on snapshot load.
> > 
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > ---
> >   hw/ppc/spapr.c         | 12 +++++++++---
> >   include/hw/ppc/spapr.h |  1 +
> >   2 files changed, 10 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index d290acfa95..55948f233f 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -1017,7 +1017,6 @@ static void spapr_dt_chosen(SpaprMachineState *spapr, void *fdt, bool reset)
> >   {
> >       MachineState *machine = MACHINE(spapr);
> >       SpaprMachineClass *smc = SPAPR_MACHINE_GET_CLASS(machine);
> > -    uint8_t rng_seed[32];
> >       int chosen;
> >   
> >       _FDT(chosen = fdt_add_subnode(fdt, 0, "chosen"));
> > @@ -1095,8 +1094,7 @@ static void spapr_dt_chosen(SpaprMachineState *spapr, void *fdt, bool reset)
> >           spapr_dt_ov5_platform_support(spapr, fdt, chosen);
> >       }
> >   
> > -    qemu_guest_getrandom_nofail(rng_seed, sizeof(rng_seed));
> > -    _FDT(fdt_setprop(fdt, chosen, "rng-seed", rng_seed, sizeof(rng_seed)));
> > +    _FDT(fdt_setprop(fdt, chosen, "rng-seed", spapr->fdt_rng_seed, 32));
> >   
> >       _FDT(spapr_dt_ovec(fdt, chosen, spapr->ov5_cas, "ibm,architecture-vec-5"));
> >   }
> > @@ -1649,6 +1647,14 @@ static void spapr_machine_reset(MachineState *machine, ShutdownCause reason)
> >       void *fdt;
> >       int rc;
> >   
> > +    if (reason != SHUTDOWN_CAUSE_SNAPSHOT_LOAD) {
> > +        /*
> > +         * Record-replay snapshot load must not consume random, this was
> > +         * already replayed from initial machine reset.
> > +         */
> > +        qemu_guest_getrandom_nofail(spapr->fdt_rng_seed, 32);
> > +    }
> > +
> >       pef_kvm_reset(machine->cgs, &error_fatal);
> >       spapr_caps_apply(spapr);
> >   
> > diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> > index f47e8419a5..f4bd204d86 100644
> > --- a/include/hw/ppc/spapr.h
> > +++ b/include/hw/ppc/spapr.h
> > @@ -204,6 +204,7 @@ struct SpaprMachineState {
> >       uint32_t fdt_size;
> >       uint32_t fdt_initial_size;
> >       void *fdt_blob;
> > +    uint8_t fdt_rng_seed[32];
> >       long kernel_size;
> >       bool kernel_le;
> >       uint64_t kernel_addr;