Re: [Qemu-devel] [PATCH v2 03/14] target/arm/monitor: Introduce qmp_query_cpu_model_expansion

[Andrew Jones]

On Wed, Jul 24, 2019 at 02:51:08PM +0200, Auger Eric wrote:
> Hi Drew,
> 
> On 6/26/19 3:26 PM, Andrew Jones wrote:
> > On Wed, Jun 26, 2019 at 09:43:09AM +0200, Auger Eric wrote:
> >> Hi Drew,
> >>
> >> On 6/21/19 6:34 PM, Andrew Jones wrote:
> >>> Add support for the query-cpu-model-expansion QMP command to Arm. We
> >>> do this selectively, only exposing CPU properties which represent
> >>> optional CPU features which the user may want to enable/disable. Also,
> >>> for simplicity, we restrict the list of queryable cpu models to 'max',
> >>> 'host', or the current type when KVM is in use, even though there
> >>> may exist KVM hosts where other types would also work. For example on a
> >>> seattle you could use 'host' for the current type, but then attempt to
> >>> query 'cortex-a57', which is also a valid CPU type to use with KVM on
> >>> seattle hosts, but that query will fail with our simplifications. This
> >>> shouldn't be an issue though as management layers and users have been
> >>> preferring the 'host' CPU type for use with KVM for quite some time.
> >>> Additionally, if the KVM-enabled QEMU instance running on a seattle
> >>> host is using the cortex-a57 CPU type, then querying 'cortex-a57' will
> >>> work. Finally, we only implement expansion type 'full', as Arm does not
> >>> yet have a "base" CPU type. Below are some example calls and results
> >>> (to save character clutter they're not in json, but are still json-ish
> >>> to give the idea)
> >>>
> >>>  # expand the 'max' CPU model
> >>>  query-cpu-model-expansion: type:full, model:{ name:max }
> >>>
> >>>  return: model:{ name:max, props:{ 'aarch64': true, 'pmu': true }}
> >>>
> >>>  # attempt to expand the 'max' CPU model with pmu=off
> >>>  query-cpu-model-expansion:
> >>>    type:full, model:{ name:max, props:{ 'pmu': false }}
> >>>
> >>>  return: model:{ name:max, props:{ 'aarch64': true, 'pmu': false }}
> >>>
> >>>  # attempt to expand the 'max' CPU model with aarch64=off
> >>>  query-cpu-model-expansion:
> >>>    type:full, model:{ name:max, props:{ 'aarch64': false }}
> >>>
> >>>  error: "'aarch64' feature cannot be disabled unless KVM is enabled
> >>>          and 32-bit EL1 is supported"
> >>>
> >>> In the last example KVM was not in use so an error was returned.
> >>>
> >>> Note1: It's possible for features to have dependencies on other
> >>> features. I.e. it may be possible to change one feature at a time
> >>> without error, but when attempting to change all features at once
> >>> an error could occur depending on the order they are processed. It's
> >>> also possible changing all at once doesn't generate an error, because
> >>> a feature's dependencies are satisfied with other features, but the
> >>> same feature cannot be changed independently without error. For these
> >>> reasons callers should always attempt to make their desired changes
> >>> all at once in order to ensure the collection is valid.
> >>>
> >>> Note2: Certainly more features may be added to the list of
> >>> advertised features, e.g. 'vfp' and 'neon'. The only requirement
> >>> is that their property set accessors fail when invalid
> >>> configurations are detected. For vfp we would need something like
> >>>
> >>>  set_vfp()
> >>>  {
> >>>    if (arm_feature(env, ARM_FEATURE_AARCH64) &&
> >>>        cpu->has_vfp != cpu->has_neon)
> >>>        error("AArch64 CPUs must have both VFP and Neon or neither")
> >>>
> >>> in its set accessor, and the same for neon, rather than doing that
> >>> check at realize time, which isn't executed at qmp query time.
> >>>
> >>> Signed-off-by: Andrew Jones <address@hidden>
> >>> ---
> >>>  qapi/target.json     |   6 +-
> >>>  target/arm/monitor.c | 132 +++++++++++++++++++++++++++++++++++++++++++
> >>>  2 files changed, 135 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/qapi/target.json b/qapi/target.json
> >>> index 1d4d54b6002e..edfa2f82b916 100644
> >>> --- a/qapi/target.json
> >>> +++ b/qapi/target.json
> >>> @@ -408,7 +408,7 @@
> >>>  ##
> >>>  { 'struct': 'CpuModelExpansionInfo',
> >>>    'data': { 'model': 'CpuModelInfo' },
> >>> -  'if': 'defined(TARGET_S390X) || defined(TARGET_I386)' }
> >>> +  'if': 'defined(TARGET_S390X) || defined(TARGET_I386) || 
> >>> defined(TARGET_ARM)' }
> >>>  
> >>>  ##
> >>>  # @query-cpu-model-expansion:
> >>> @@ -433,7 +433,7 @@
> >>>  #   query-cpu-model-expansion while using these is not advised.
> >>>  #
> >>>  # Some architectures may not support all expansion types. s390x supports
> >>> -# "full" and "static".
> >>> +# "full" and "static". Arm only supports "full".
> >>>  #
> >>>  # Returns: a CpuModelExpansionInfo. Returns an error if expanding CPU 
> >>> models is
> >>>  #          not supported, if the model cannot be expanded, if the model 
> >>> contains
> >>> @@ -447,7 +447,7 @@
> >>>    'data': { 'type': 'CpuModelExpansionType',
> >>>              'model': 'CpuModelInfo' },
> >>>    'returns': 'CpuModelExpansionInfo',
> >>> -  'if': 'defined(TARGET_S390X) || defined(TARGET_I386)' }
> >>> +  'if': 'defined(TARGET_S390X) || defined(TARGET_I386) || 
> >>> defined(TARGET_ARM)' }
> >>>  
> >>>  ##
> >>>  # @CpuDefinitionInfo:
> >>> diff --git a/target/arm/monitor.c b/target/arm/monitor.c
> >>> index 41b32b94b258..19e3120eef95 100644
> >>> --- a/target/arm/monitor.c
> >>> +++ b/target/arm/monitor.c
> >>> @@ -23,7 +23,13 @@
> >>>  #include "qemu/osdep.h"
> >>>  #include "hw/boards.h"
> >>>  #include "kvm_arm.h"
> >>> +#include "qapi/error.h"
> >>> +#include "qapi/visitor.h"
> >>> +#include "qapi/qobject-input-visitor.h"
> >>>  #include "qapi/qapi-commands-target.h"
> >>> +#include "qapi/qmp/qerror.h"
> >>> +#include "qapi/qmp/qdict.h"
> >>> +#include "qom/qom-qobject.h"
> >>>  
> >>>  static GICCapability *gic_cap_new(int version)
> >>>  {
> >>> @@ -82,3 +88,129 @@ GICCapabilityList *qmp_query_gic_capabilities(Error 
> >>> **errp)
> >>>  
> >>>      return head;
> >>>  }
> >>> +
> >>> +static const char *cpu_model_advertised_features[] = {
> >>> +    "aarch64", "pmu",
> >>> +    NULL
> >>> +};
> >>> +
> >>> +CpuModelExpansionInfo 
> >>> *qmp_query_cpu_model_expansion(CpuModelExpansionType type,
> >>> +                                                     CpuModelInfo *model,
> >>> +                                                     Error **errp)
> >>> +{
> >>> +    CpuModelExpansionInfo *expansion_info;
> >>> +    const QDict *qdict_in = NULL;
> >>> +    QDict *qdict_out;
> >>> +    ObjectClass *oc;
> >>> +    Object *obj;
> >>> +    const char *name;
> >>> +    int i;
> >>> +
> >>> +    if (type != CPU_MODEL_EXPANSION_TYPE_FULL) {
> >>> +        error_setg(errp, "The requested expansion type is not 
> >>> supported.");
> >>> +        return NULL;
> >>> +    }
> >>> +
> >>> +    if (!kvm_enabled() && !strcmp(model->name, "host")) {
> >>> +        error_setg(errp, "The CPU definition '%s' requires KVM", 
> >>> model->name);
> >>> +        return NULL;
> >>> +    }
> >>> +
> >>> +    oc = cpu_class_by_name(TYPE_ARM_CPU, model->name);
> >>> +    if (!oc) {
> >>> +        error_setg(errp, "The CPU definition '%s' is unknown.", 
> >>> model->name);
> >>> +        return NULL;
> >>> +    }
> >>> +
> >>> +    if (kvm_enabled()) {
> >>> +        const char *cpu_type = current_machine->cpu_type;
> >>> +        int len = strlen(cpu_type) - strlen(ARM_CPU_TYPE_SUFFIX);
> >>> +        bool supported = false;
> >>> +
> >>> +        if (!strcmp(model->name, "host") || !strcmp(model->name, "max")) 
> >>> {
> >>> +            /* These are kvmarm's recommended cpu types */
> >>> +            supported = true;
> >>> +        } else if (strlen(model->name) == len &&
> >>> +                   !strncmp(model->name, cpu_type, len)) {
> >>> +            /* KVM is enabled and we're using this type, so it works. */
> >>> +            supported = true;
> >>> +        }
> >>> +        if (!supported) {
> >>> +            error_setg(errp, "The CPU definition '%s' cannot "
> >> use model name instead of CPU definition?
> > 
> > I took that wording from s390x, but maybe I prefer "The CPU type..."
> > better. I'll change it for v3.
> This CPU type is not recognized as an ARM CPU type?

That's not what this error message is stating. The CPU type may well be an
ARM CPU type, but it's not one you can expect to use with KVM enabled. I
currently have

  "The CPU type '%s' cannot "
  "be used with KVM on this host", model->name)

queued up for v3.

> > 
> >>> +                             "be used with KVM on this host", 
> >>> model->name);
> >>
> >> According to your commit mesg doesn't it mean that we fall into the
> >> simplification you mentionned and not necessarily that the model name
> >> cannot be used along with KVM?
> > 
> > There's no way to know that. The simplification is meant to avoid having
> > to know which models will work with KVM, because most don't, but some do.
> > Can you suggest wording you'd prefer if you don't want to make the error
> > message so absolute? I think I prefer keeping it simple like this and
> > just saying it doesn't work.
> Something like:
> "We cannot guarantee the CPU type %s works with KVM on this host"

OK, I can change to this one.

> > 
> >>
> >>> seattle you could use 'host' for the current type, but then attempt to
> >>> query 'cortex-a57', which is also a valid CPU type to use with KVM on
> >>> seattle hosts, but that query will fail with our simplifications
> >>> +            return NULL;
> >>> +        }
> >>> +    }
> >>> +
> >>> +    if (model->props) {
> >>> +        qdict_in = qobject_to(QDict, model->props);
> >>> +        if (!qdict_in) {
> >>> +            error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", 
> >>> "dict");
> >>> +            return NULL;
> >>> +        }
> >>> +    }
> >>> +
> >>> +    obj = object_new(object_class_get_name(oc));
> >>> +
> >>> +    if (qdict_in) {
> >>> +        Visitor *visitor;
> >>> +
> >>> +        visitor = qobject_input_visitor_new(model->props);
> >>> +        visit_start_struct(visitor, NULL, NULL, 0, errp);
> >>> +        if (*errp) {
> >> Normally we shouldn't do that as errp can be NULL. see 
> >> /include/qapi/error.h
> >> I see the same in cpu_model_from_info() by the way (s390x/cpu_models.c)
> >> Maybe you can guarantee that errp isn't NULL but ...
> > 
> > Yeah, I know about the errp NULL thing, which is why I use local_err
> > elsewhere. I decided to follow s390x here though because I'm guessing
> > our QMP function will never be called with a NULL errp, it just
> > wouldn't work that way. Would you be satisfied with an assert(errp)
> > at the top of the function? Or should I switch all these to local_err
> > and then propagate?
> well up to maintainers. If it is not that much a pain, just propagate ;-)
> > 

OK, I'll just propagate.

Thanks,
drew