Re: [PATCH 03/16] target/riscv/cpu.c: restrict 'mvendorid' value

[Daniel Henrique Barboza]

On 6/12/23 00:56, Alistair Francis wrote:
> On Wed, May 31, 2023 at 5:49 AM Daniel Henrique Barboza
> <dbarboza@ventanamicro.com> wrote:
> > We're going to change the handling of mvendorid/marchid/mimpid by the
> > KVM driver. Since these are always present in all CPUs let's put the
> > same validation for everyone.
> >
> > It doesn't make sense to allow 'mvendorid' to be different than it
> > is already set in named (vendor) CPUs. Generic (dynamic) CPUs can have
> > any 'mvendorid' they want.
> >
> > Change 'mvendorid' to be a class property created via
> > 'object_class_property_add', instead of using the DEFINE_PROP_UINT32()
> > macro. This allow us to define a custom setter for it that will verify,
> > for named CPUs, if mvendorid is different than it is already set by the
> > CPU. This is the error thrown for the 'veyron-v1' CPU if 'mvendorid' is
> > set to an invalid value:
> >
> > $ qemu-system-riscv64 -M virt -nographic -cpu veyron-v1,mvendorid=2
> > qemu-system-riscv64: can't apply global veyron-v1-riscv-cpu.mvendorid=2:
> >      Unable to change veyron-v1-riscv-cpu mvendorid (0x61f)
>
> Is this something we want to enforce? What if someone wanted to test
> using the veyron-v1 CPU but they wanted to change some properties. I
> don't see an advantage in not letting them do that

The idea is to keep things simpler for us. As it is today we forbid users to
enable/disable extensions for vendor CPUs. Doing the same thing for
mvendorid/marchid/mimpid seems consistent with what we're already doing.

Also, guest software might rely on vendor IDs from the CPU to take certain
actions, and if the user is free to change the CPU ID from vendor CPUs the
software will misbehave and the user can claim "I created a veyron-v1 CPU and
the guest it's like like it's not". Allowing mvendorid and friends to be changed
doesn't do much for users (we forbid enabling/disabling extensions, so what's
to gain from changing machine IDs?) and it can be a potential source of bugs.



Thanks,


Daniel


> Alistair
>
> > Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> > ---
> >   target/riscv/cpu.c | 31 ++++++++++++++++++++++++++++++-
> >   1 file changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> > index 72f5433776..bcd69bb032 100644
> > --- a/target/riscv/cpu.c
> > +++ b/target/riscv/cpu.c
> > @@ -1723,7 +1723,6 @@ static void riscv_cpu_add_user_properties(Object *obj)
> >   static Property riscv_cpu_properties[] = {
> >       DEFINE_PROP_BOOL("debug", RISCVCPU, cfg.debug, true),
> >
> > -    DEFINE_PROP_UINT32("mvendorid", RISCVCPU, cfg.mvendorid, 0),
> >       DEFINE_PROP_UINT64("marchid", RISCVCPU, cfg.marchid, RISCV_CPU_MARCHID),
> >       DEFINE_PROP_UINT64("mimpid", RISCVCPU, cfg.mimpid, RISCV_CPU_MIMPID),
> >
> > @@ -1810,6 +1809,32 @@ static const struct TCGCPUOps riscv_tcg_ops = {
> >   #endif /* !CONFIG_USER_ONLY */
> >   };
> >
> > +static bool riscv_cpu_is_dynamic(Object *cpu_obj)
> > +{
> > +    return object_dynamic_cast(cpu_obj, TYPE_RISCV_DYNAMIC_CPU) != NULL;
> > +}
> > +
> > +static void cpu_set_mvendorid(Object *obj, Visitor *v, const char *name,
> > +                              void *opaque, Error **errp)
> > +{
> > +    bool dynamic_cpu = riscv_cpu_is_dynamic(obj);
> > +    RISCVCPU *cpu = RISCV_CPU(obj);
> > +    uint32_t prev_val = cpu->cfg.mvendorid;
> > +    uint32_t value;
> > +
> > +    if (!visit_type_uint32(v, name, &value, errp)) {
> > +        return;
> > +    }
> > +
> > +    if (!dynamic_cpu && prev_val != value) {
> > +        error_setg(errp, "Unable to change %s mvendorid (0x%x)",
> > +                   object_get_typename(obj), prev_val);
> > +        return;
> > +    }
> > +
> > +    cpu->cfg.mvendorid = value;
> > +}
> > +
> >   static void riscv_cpu_class_init(ObjectClass *c, void *data)
> >   {
> >       RISCVCPUClass *mcc = RISCV_CPU_CLASS(c);
> > @@ -1841,6 +1866,10 @@ static void riscv_cpu_class_init(ObjectClass *c, void 
> > *data)
> >       cc->gdb_get_dynamic_xml = riscv_gdb_get_dynamic_xml;
> >       cc->tcg_ops = &riscv_tcg_ops;
> >
> > +    object_class_property_add(c, "mvendorid", "uint32", NULL,
> > +                              cpu_set_mvendorid,
> > +                              NULL, NULL);
> > +
> >       device_class_set_props(dc, riscv_cpu_properties);
> >   }
> >
> > --
> > 2.40.1