Re: [PATCH v3 16/19] target/riscv/cpu.c: create KVM mock properties

[Andrew Jones <ajones@ventanamicro.com>]

On Thu, Jun 22, 2023 at 10:56:57AM -0300, Daniel Henrique Barboza wrote:
> KVM-specific properties are being created inside target/riscv/kvm.c. But
> at this moment we're gathering all the remaining properties from TCG and
> adding them as is when running KVM. This creates a situation where
> non-KVM properties are setting flags to 'true' due to its default
> settings (e.g.  Zawrs). Users can also freely enable them via command
> line.
> 
> This doesn't impact runtime per se because KVM doesn't care about these
> flags, but code such as riscv_isa_string_ext() take those flags into
> account. The result is that, for a KVM guest, setting non-KVM properties
> will make them appear in the riscv,isa DT.
> 
> We want to keep the same API for both TCG and KVM and at the same time,
> when running KVM, forbid non-KVM extensions to be enabled internally. We
> accomplish both by changing riscv_cpu_add_user_properties() to add a
> mock/no-op boolean property for every non-KVM extension in
> riscv_cpu_extensions[]. Then, when running KVM, users are still free to
> set extensions at will, we'll treat non-KVM extensions as a no-op, and
> riscv_isa_string_ext() will not report bogus extensions in the DT.
> 
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>  target/riscv/cpu.c | 36 +++++++++++++++++++++++++++++++++---
>  1 file changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index b65db165cc..f5209f0789 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -1720,6 +1720,18 @@ static Property riscv_cpu_extensions[] = {
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> +
> +static void cpu_set_cfg_noop(Object *obj, Visitor *v,
> +                             const char *name,
> +                             void *opaque, Error **errp)
> +{
> +    bool value;
> +
> +    if (!visit_type_bool(v, name, &value, errp)) {
> +        return;
> +    }
> +}
> +
>  /*
>   * Add CPU properties with user-facing flags.
>   *
> @@ -1738,9 +1750,27 @@ static void riscv_cpu_add_user_properties(Object *obj)
>      riscv_cpu_add_misa_properties(obj);
>  
>      for (prop = riscv_cpu_extensions; prop && prop->name; prop++) {
> -        /* Check if KVM didn't create the property already */
> -        if (object_property_find(obj, prop->name)) {
> -            continue;
> +        if (riscv_running_kvm()) {
> +            /* Check if KVM didn't create the property already */
> +            if (object_property_find(obj, prop->name)) {
> +                continue;
> +            }
> +
> +            /*
> +             * Set every multi-letter extension that KVM doesn't
> +             * know as a no-op. This will allow users to set values
> +             * to them while keeping their internal state to 'false'.
> +             *
> +             * We're giving a pass for non-bool properties since they're
> +             * not related to the availability of extensions and can be
> +             * safely ignored as is.
> +             */
> +            if (prop->info == &qdev_prop_bool) {
> +                object_property_add(obj, prop->name, "bool",
> +                                    NULL, cpu_set_cfg_noop,
> +                                    NULL, NULL);
> +                continue;
> +            }
>          }
>  
>          qdev_property_add_static(dev, prop);
> -- 
> 2.41.0
>

I think we should actually fail with an error when the user tries to
enable an extension KVM doesn't support. Otherwise a user may be
confused as to why their Zawrs=on didn't provide them a machine with
Zawrs. And, when KVM learns how to provide that support to guests
(Zawrs is actually on my TODO...), then migrating the same VM to
later KVM/QEMU will actually enable the feature, possibly confusing
the guest.

So, we should probably just not add any extension properties to KVM
guests which can't be enabled. Then, as we add support to KVM, we'll
add the properties too.

Thanks,
drew