Re: [PATCH V2 08/11] target/i386: Clean up LBR format handling

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 1/29/2026 7:09 AM, Zide Chen wrote:
> Since the lbr-fmt property is masked with PERF_CAP_LBR_FMT in
> DEFINE_PROP_UINT64_CHECKMASK(), there is no need to explicitly validate
> user-requested lbr-fmt values.
>
> The PMU feature is only supported when running under KVM, so initialize
> cpu->lbr_fmt in kvm_cpu_instance_init().  Use -1 as the default lbr-fmt,
> rather than initializing it with ~PERF_CAP_LBR_FMT, which is misleading
> as it suggests a semantic relationship that does not exist.
>
> Rename requested_lbr_fmt to a more generic guest_fmt.  When lbr-fmt is
> not specified and cpu->migratable is false, the guest lbr_fmt value is
> not user-requested.
>
> Signed-off-by: Zide Chen <zide.chen@intel.com>
> ---
> V2:
> - New patch.
>
>  target/i386/cpu.c         | 18 ++++++------------
>  target/i386/kvm/kvm-cpu.c |  2 ++
>  2 files changed, 8 insertions(+), 12 deletions(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index f2c83b4f259c..09180c718d58 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -9788,7 +9788,7 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
>      X86CPUClass *xcc = X86_CPU_GET_CLASS(dev);
>      CPUX86State *env = &cpu->env;
>      Error *local_err = NULL;
> -    unsigned requested_lbr_fmt;
> +    unsigned guest_fmt;
>  
>      if (!kvm_enabled())
>          cpu->enable_pmu = false;
> @@ -9828,11 +9828,7 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
>       * Override env->features[FEAT_PERF_CAPABILITIES].LBR_FMT
>       * with user-provided setting.
>       */
> -    if (cpu->lbr_fmt != ~PERF_CAP_LBR_FMT) {
> -        if ((cpu->lbr_fmt & PERF_CAP_LBR_FMT) != cpu->lbr_fmt) {
> -            error_setg(errp, "invalid lbr-fmt");
> -            return;
> -        }
> +    if (cpu->lbr_fmt != -1) {
>          env->features[FEAT_PERF_CAPABILITIES] &= ~PERF_CAP_LBR_FMT;
>          env->features[FEAT_PERF_CAPABILITIES] |= cpu->lbr_fmt;
>      }
> @@ -9841,9 +9837,8 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
>       * vPMU LBR is supported when 1) KVM is enabled 2) Option pmu=on and
>       * 3)vPMU LBR format matches that of host setting.
>       */
> -    requested_lbr_fmt =
> -        env->features[FEAT_PERF_CAPABILITIES] & PERF_CAP_LBR_FMT;
> -    if (requested_lbr_fmt && kvm_enabled()) {
> +    guest_fmt = env->features[FEAT_PERF_CAPABILITIES] & PERF_CAP_LBR_FMT;
> +    if (guest_fmt) {
>          uint64_t host_perf_cap =
>              x86_cpu_get_supported_feature_word(NULL, FEAT_PERF_CAPABILITIES);
>          unsigned host_lbr_fmt = host_perf_cap & PERF_CAP_LBR_FMT;
> @@ -9852,10 +9847,10 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
>              error_setg(errp, "vPMU: LBR is unsupported without pmu=on");
>              return;
>          }
> -        if (requested_lbr_fmt != host_lbr_fmt) {
> +        if (guest_fmt != host_lbr_fmt) {
>              error_setg(errp, "vPMU: the lbr-fmt value (0x%x) does not match "
>                          "the host value (0x%x).",
> -                        requested_lbr_fmt, host_lbr_fmt);
> +                        guest_fmt, host_lbr_fmt);
>              return;
>          }
>      }
> @@ -10279,7 +10274,6 @@ static void x86_cpu_initfn(Object *obj)
>      object_property_add_alias(obj, "sse4_2", obj, "sse4.2");
>  
>      object_property_add_alias(obj, "hv-apicv", obj, "hv-avic");
> -    cpu->lbr_fmt = ~PERF_CAP_LBR_FMT;
>      object_property_add_alias(obj, "lbr_fmt", obj, "lbr-fmt");
>  
>      if (xcc->model) {
> diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
> index 33a8c26bc27c..b4500ab69f82 100644
> --- a/target/i386/kvm/kvm-cpu.c
> +++ b/target/i386/kvm/kvm-cpu.c
> @@ -231,6 +231,8 @@ static void kvm_cpu_instance_init(CPUState *cs)
>          kvm_cpu_max_instance_init(cpu);
>      }
>  
> +    cpu->lbr_fmt = -1;
> +
>      kvm_cpu_xsave_init();
>  }
>  

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>