Re: [PATCH v14] qapi: introduce 'query-x86-cpuid' QMP command.

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Aug 10, 2021 at 09:51:31AM +0300, Valeriy Vdovin wrote:
> From: Valeriy Vdovin <valeriy.vdovin@virtuozzo.com>
> 
> Introducing new QMP command 'query-x86-cpuid'. This command can be used to
> get virtualized cpu model info generated by QEMU during VM initialization in
> the form of cpuid representation.
> 
> Diving into more details about virtual CPU generation: QEMU first parses '-cpu'
> command line option. From there it takes the name of the model as the basis for
> feature set of the new virtual CPU. After that it uses trailing '-cpu' options,
> that state if additional cpu features should be present on the virtual CPU or
> excluded from it (tokens '+'/'-' or '=on'/'=off').
> After that QEMU checks if the host's cpu can actually support the derived
> feature set and applies host limitations to it.
> After this initialization procedure, virtual CPU has it's model and
> vendor names, and a working feature set and is ready for identification
> instructions such as CPUID.
> 
> To learn exactly how virtual CPU is presented to the guest machine via CPUID
> instruction, new QMP command can be used. By calling 'query-x86-cpuid'
> command, one can get a full listing of all CPUID leaves with subleaves which are
> supported by the initialized virtual CPU.
> 
> Other than debug, the command is useful in cases when we would like to
> utilize QEMU's virtual CPU initialization routines and put the retrieved
> values into kernel CPUID overriding mechanics for more precise control
> over how various processes perceive its underlying hardware with
> container processes as a good example.
> 
> The command is specific to x86. It is currenly only implemented for KVM acceleator.
> 
> Output format:
> The output is a plain list of leaf/subleaf argument combinations, that
> return 4 words in registers EAX, EBX, ECX, EDX.
> 
> Use example:
> qmp_request: {
>   "execute": "query-x86-cpuid"
> }
> 
> qmp_response: {
>   "return": [
>     {
>       "eax": 1073741825,
>       "edx": 77,
>       "in-eax": 1073741824,
>       "ecx": 1447775574,
>       "ebx": 1263359563
>     },
>     {
>       "eax": 16777339,
>       "edx": 0,
>       "in-eax": 1073741825,
>       "ecx": 0,
>       "ebx": 0
>     },
>     {
>       "eax": 13,
>       "edx": 1231384169,
>       "in-eax": 0,
>       "ecx": 1818588270,
>       "ebx": 1970169159
>     },
>     {
>       "eax": 198354,
>       "edx": 126614527,
>       "in-eax": 1,
>       "ecx": 2176328193,
>       "ebx": 2048
>     },
>     ....
>     {
>       "eax": 12328,
>       "edx": 0,
>       "in-eax": 2147483656,
>       "ecx": 0,
>       "ebx": 0
>     }
>   ]
> }
> 
> Signed-off-by: Valeriy Vdovin <valeriy.vdovin@virtuozzo.com>
> ---
> v2: - Removed leaf/subleaf iterators.
>     - Modified cpu_x86_cpuid to return false in cases when count is
>       greater than supported subleaves.
> v3: - Fixed structure name coding style.
>     - Added more comments
>     - Ensured buildability for non-x86 targets.
> v4: - Fixed cpu_x86_cpuid return value logic and handling of 0xA leaf.
>     - Fixed comments.
>     - Removed target check in qmp_query_cpu_model_cpuid.
> v5: - Added error handling code in qmp_query_cpu_model_cpuid
> v6: - Fixed error handling code. Added method to query_error_class
> v7: - Changed implementation in favor of cached cpuid_data for
>       KVM_SET_CPUID2
> v8: - Renamed qmp method to query-kvm-cpuid and some fields in response.
>     - Modified documentation to qmp method
>     - Removed helper struct declaration
> v9: - Renamed 'in_eax' / 'in_ecx' fields to 'in-eax' / 'in-ecx'
>     - Pasted more complete response to commit message.
> v10:
>     - Subject changed
>     - Fixes in commit message
>     - Small fixes in QMP command docs
> v11:
>     - Added explanation about CONFIG_KVM to the commit message.
> v12:
>     - Changed title from query-kvm-cpuid to query-x86-cpuid
>     - Removed CONFIG_KVM ifdefs
>     - Added detailed error messages for some stub/unimplemented cases.
> v13:
>     - Tagged with since 6.2
> v14:
>     - Rebased to latest master 632eda54043d6f26ff87dac16233e14b4708b967
>     - Added note about error return cases in api documentation.
> 
>  qapi/machine-target.json   | 46 ++++++++++++++++++++++++++++++++++
>  softmmu/cpus.c             |  2 +-
>  target/i386/kvm/kvm-stub.c | 10 ++++++++
>  target/i386/kvm/kvm.c      | 51 ++++++++++++++++++++++++++++++++++++++
>  tests/qtest/qmp-cmd-test.c |  1 +
>  5 files changed, 109 insertions(+), 1 deletion(-)
> 
> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> index e7811654b7..71648a4f56 100644
> --- a/qapi/machine-target.json
> +++ b/qapi/machine-target.json
> @@ -329,3 +329,49 @@
>  ##
>  { 'command': 'query-cpu-definitions', 'returns': ['CpuDefinitionInfo'],
>    'if': 'defined(TARGET_PPC) || defined(TARGET_ARM) || defined(TARGET_I386) || defined(TARGET_S390X) || defined(TARGET_MIPS)' }
> +
> +##
> +# @CpuidEntry:
> +#
> +# A single entry of a CPUID response.
> +#
> +# One entry holds full set of information (leaf) returned to the guest
> +# in response to it calling a CPUID instruction with eax, ecx used as
> +# the arguments to that instruction. ecx is an optional argument as
> +# not all of the leaves support it.
> +#
> +# @in-eax: CPUID argument in eax
> +# @in-ecx: CPUID argument in ecx
> +# @eax: CPUID result in eax
> +# @ebx: CPUID result in ebx
> +# @ecx: CPUID result in ecx
> +# @edx: CPUID result in edx
> +#
> +# Since: 6.2
> +##
> +{ 'struct': 'CpuidEntry',
> +  'data': { 'in-eax' : 'uint32',
> +            '*in-ecx' : 'uint32',
> +            'eax' : 'uint32',
> +            'ebx' : 'uint32',
> +            'ecx' : 'uint32',
> +            'edx' : 'uint32'
> +          },
> +  'if': 'defined(TARGET_I386)' }
> +
> +##
> +# @query-x86-cpuid:
> +#
> +# Returns raw data from the emulated CPUID table for the first VCPU.
> +# The emulated CPUID table defines the response to the CPUID
> +# instruction when executed by the guest operating system.
> +# 
> +#
> +# Returns: a list of CpuidEntry. Returns error when qemu is configured with
> +# --disable-kvm flag or if qemu is run with any other accelerator than KVM.
> +#
> +# Since: 6.2
> +##
> +{ 'command': 'query-x86-cpuid',
> +  'returns': ['CpuidEntry'],
> +  'if': 'defined(TARGET_I386)' }
> diff --git a/softmmu/cpus.c b/softmmu/cpus.c
> index 071085f840..8501081897 100644
> --- a/softmmu/cpus.c
> +++ b/softmmu/cpus.c
> @@ -129,7 +129,7 @@ void hw_error(const char *fmt, ...)
>  /*
>   * The chosen accelerator is supposed to register this.
>   */
> -static const AccelOpsClass *cpus_accel;
> +const AccelOpsClass *cpus_accel;
>  
>  void cpu_synchronize_all_states(void)
>  {
> diff --git a/target/i386/kvm/kvm-stub.c b/target/i386/kvm/kvm-stub.c
> index f6e7e4466e..9eb04d908f 100644
> --- a/target/i386/kvm/kvm-stub.c
> +++ b/target/i386/kvm/kvm-stub.c
> @@ -12,6 +12,7 @@
>  #include "qemu/osdep.h"
>  #include "cpu.h"
>  #include "kvm_i386.h"
> +#include "qapi/error.h"
>  
>  #ifndef __OPTIMIZE__
>  bool kvm_has_smm(void)
> @@ -44,3 +45,12 @@ bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp)
>  {
>      abort();
>  }
> +
> +typedef struct CpuidEntryList CpuidEntryList;
> +CpuidEntryList *qmp_query_x86_cpuid(Error **errp);
> +
> +CpuidEntryList *qmp_query_x86_cpuid(Error **errp)
> +{
> +    error_setg(errp, "Not implemented in --disable-kvm configuration");
> +    return NULL;
> +}
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index e69abe48e3..28e79cd0cf 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -20,11 +20,13 @@
>  
>  #include <linux/kvm.h>
>  #include "standard-headers/asm-x86/kvm_para.h"
> +#include "qapi/qapi-commands-machine-target.h"
>  
>  #include "cpu.h"
>  #include "host-cpu.h"
>  #include "sysemu/sysemu.h"
>  #include "sysemu/hw_accel.h"
> +#include "sysemu/accel-ops.h"
>  #include "sysemu/kvm_int.h"
>  #include "sysemu/runstate.h"
>  #include "kvm_i386.h"
> @@ -1540,6 +1542,51 @@ static Error *invtsc_mig_blocker;
>  
>  #define KVM_MAX_CPUID_ENTRIES  100
>  
> +struct kvm_cpuid2 *cpuid_data_cached;
> +extern const AccelOpsClass *cpus_accel;
> +
> +static inline int is_kvm_accel(AccelOpsClass *class)
> +{
> +    ObjectClass *parent_class;

We normally call ObjectClass variables 'oc'.

> +
> +    parent_class = &class->parent_class;

The right way to cast a class/object to a different type is to
use the type casting macros. e.g.:

    oc = OBJECT_CLASS(class);

> +    return strcmp(object_class_get_name(parent_class),
> +        "kvm-accel-ops") == 0;

You should never do a string comparison of the QOM type name to
decide what to do.  If you want to know if a class implements a
specific type, you can just call:

   object_class_dynamic_cast(class, TYPE_KVM_ACCEL)

In either case, this function can probably be deleted and
replaced with a simple kvm_enabled() call (see below).

> +}
> +
> +CpuidEntryList *qmp_query_x86_cpuid(Error **errp)
> +{
> +    int i;
> +    struct kvm_cpuid_entry2 *kvm_entry;
> +    CpuidEntryList *head = NULL, **tail = &head;
> +    CpuidEntry *entry;
> +
> +    if (!cpuid_data_cached) {
> +         if (cpus_accel && !is_kvm_accel((AccelOpsClass *)cpus_accel))

This can be replaced by a kvm_enabled() check.

Also, I suggest avoid nesting conditionals when possible.  e.g.:

  if (!kvm_enabled()) {
      error_setg(...);
      return NULL;
  }
  if (!cpuid_data_cached) {
      error_setg(...);
      return NULL;
  }

> +             error_setg(errp, "Not implemented for non-kvm accel");
> +         else
> +             error_setg(errp, "VCPU was not initialized yet");
> +         return NULL;
> +    }
> +
> +    for (i = 0; i < cpuid_data_cached->nent; ++i) {
> +        kvm_entry = &cpuid_data_cached->entries[i];
> +        entry = g_malloc0(sizeof(*entry));
> +        entry->in_eax = kvm_entry->function;
> +        if (kvm_entry->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX) {
> +            entry->in_ecx = kvm_entry->index;
> +            entry->has_in_ecx = true;
> +        }
> +        entry->eax = kvm_entry->eax;
> +        entry->ebx = kvm_entry->ebx;
> +        entry->ecx = kvm_entry->ecx;
> +        entry->edx = kvm_entry->edx;
> +        QAPI_LIST_APPEND(tail, entry);
> +    }
> +
> +    return head;
> +}
> +
>  int kvm_arch_init_vcpu(CPUState *cs)
>  {
>      struct {
> @@ -1923,6 +1970,10 @@ int kvm_arch_init_vcpu(CPUState *cs)
>      if (r) {
>          goto fail;
>      }
> +    if (!cpuid_data_cached) {
> +        cpuid_data_cached = g_malloc0(sizeof(cpuid_data));
> +        memcpy(cpuid_data_cached, &cpuid_data, sizeof(cpuid_data));
> +    }
>  
>      if (has_xsave) {
>          env->xsave_buf_len = sizeof(struct kvm_xsave);
> diff --git a/tests/qtest/qmp-cmd-test.c b/tests/qtest/qmp-cmd-test.c
> index c98b78d033..bd883f7f52 100644
> --- a/tests/qtest/qmp-cmd-test.c
> +++ b/tests/qtest/qmp-cmd-test.c
> @@ -46,6 +46,7 @@ static int query_error_class(const char *cmd)
>          { "query-balloon", ERROR_CLASS_DEVICE_NOT_ACTIVE },
>          { "query-hotpluggable-cpus", ERROR_CLASS_GENERIC_ERROR },
>          { "query-vm-generation-id", ERROR_CLASS_GENERIC_ERROR },
> +        { "query-x86-cpuid", ERROR_CLASS_GENERIC_ERROR },
>          { NULL, -1 }
>      };
>      int i;
> -- 
> 2.17.1
> 

-- 
Eduardo