Re: [RFC PATCH-for-9.1 14/21] system: Introduce QMP generic_query_cpu_definitions()

[Philippe Mathieu-Daudé]

Hi Markus,

On 26/3/24 14:28, Markus Armbruster wrote:
> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
>
> > Each target use a common template for qmp_query_cpu_definitions().
> >
> > Extract it as generic_query_cpu_definitions(), keeping the
> > target-specific implementations as the following SysemuCPUOps
> > handlers:
> >   - cpu_list_compare()
> >   - add_definition()
> >   - add_alias_definitions()
> >
> > Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> > ---
> >   MAINTAINERS                           |  2 +
> >   include/hw/core/sysemu-cpu-ops.h      | 14 ++++++
> >   include/qapi/commands-target-compat.h | 14 ++++++
> >   system/cpu-qmp-cmds.c                 | 71 +++++++++++++++++++++++++++
> >   system/meson.build                    |  1 +
> >   5 files changed, 102 insertions(+)
> >   create mode 100644 include/qapi/commands-target-compat.h
> >   create mode 100644 system/cpu-qmp-cmds.c


> > diff --git a/system/cpu-qmp-cmds.c b/system/cpu-qmp-cmds.c
> > new file mode 100644
> > index 0000000000..daeb131159
> > --- /dev/null
> > +++ b/system/cpu-qmp-cmds.c
> > @@ -0,0 +1,71 @@
> > +/*
> > + * QAPI helpers for target specific QMP commands
> > + *
> > + * SPDX-FileCopyrightText: 2024 Linaro Ltd.
> > + * SPDX-License-Identifier: GPL-2.0-or-later
> > + */
> > +
> > +#include "qemu/osdep.h"
> > +#include "qom/object.h"
> > +#include "qapi/commands-target-compat.h"
> > +#include "sysemu/arch_init.h"
> > +#include "hw/core/cpu.h"
> > +#include "hw/core/sysemu-cpu-ops.h"
> > +
> > +static void cpu_common_add_definition(gpointer data, gpointer user_data)
> > +{
> > +    ObjectClass *oc = data;
> > +    CpuDefinitionInfoList **cpu_list = user_data;
> > +    CpuDefinitionInfo *info;
> > +    const char *typename;
> > +
> > +    typename = object_class_get_name(oc);
> > +    info = g_malloc0(sizeof(*info));
> > +    info->name = cpu_model_from_type(typename);
> > +    info->q_typename = g_strdup(typename);
> > +
> > +    QAPI_LIST_PREPEND(*cpu_list, info);
> > +}
> > +
> > +static void arch_add_cpu_definitions(CpuDefinitionInfoList **cpu_list,
> > +                                     const char *cpu_typename)
> > +{
> > +    ObjectClass *oc;
> > +    GSList *list;
> > +    const struct SysemuCPUOps *ops;
> > +
> > +    oc = object_class_by_name(cpu_typename);
> > +    if (!oc) {
> > +        return;
> > +    }
> > +    ops = CPU_CLASS(oc)->sysemu_ops;
> > +
> > +    list = object_class_get_list(cpu_typename, false);
> > +    if (ops->cpu_list_compare) {
> > +        list = g_slist_sort(list, ops->cpu_list_compare);
> > +    }
> > +    g_slist_foreach(list, ops->add_definition ? : cpu_common_add_definition,
> > +                    cpu_list);
> > +    g_slist_free(list);
> > +
> > +    if (ops->add_alias_definitions) {
> > +        ops->add_alias_definitions(cpu_list);
> > +    }
> > +}
> > +
> > +CpuDefinitionInfoList *generic_query_cpu_definitions(Error **errp)
> > +{
> > +    CpuDefinitionInfoList *cpu_list = NULL;
> > +
> > +    for (unsigned i = 0; i <= QEMU_ARCH_BIT_LAST; i++) {
> > +        const char *cpu_typename;
> > +
> > +        cpu_typename = cpu_typename_by_arch_bit(i);
> > +        if (!cpu_typename) {
> > +            continue;
> > +        }
> > +        arch_add_cpu_definitions(&cpu_list, cpu_typename);
> > +    }
> > +
> > +    return cpu_list;
> > +}
>
> The target-specific qmp_query_cpu_definitions() this is going to replace
> each execute the equivalent of *one* loop iteration: the one
> corresponding to their own arch bit.
>
> For the replacement to be faithful, as cpu_typename_by_arch_bit() must
> return non-null exactly once.
>
> This is the case for the qemu-system-TARGET.  The solution feels
> overengineered there.
>
> I figure cpu_typename_by_arch_bit() will return non-null multiple times
> in a future single binary supporting heterogeneous machines.
>
> Such a single binary then can't serve as drop-in replacement for the
> qemu-system-TARGET, because query-cpu-definitions returns more.
>
> To get a drop-in replacement, we'll need additional logic to restrict
> the query for the homogeneous use case.

Can we ask the management layer to provide the current homogeneous
target via argument? Otherwise we can add a new query-cpu-definitions-v2
command requiring an explicit target argument, allowing 'all', and
deprecate the current query-cpu-definitions.

> I think this needs to be discussed in the commit message.
>
> Possibly easier: don't loop over the bits, relying on
> cpu_typename_by_arch_bit() to select the right one.  Instead get the
> right bit from somewhere.
>
> We can switch to a loop when we need it for the heterogeneous case.

Alex suggested to consider heterogeneous emulation the new default,
and the current homogeneous use as a particular case. I'd rather not
plan on a "heterogeneous switch day" and get things integrated in
the way, otherwise we'll never get there...

Regards,

Phil.