Re: [PATCH] target/i386: add -cpu,lbr=true support to enable guest LBR

[Eduardo Habkost]

I've just noticed this on my review queue (apologies for the long
delay).  Comments below:

On Sun, Jul 26, 2020 at 11:32:20PM +0800, Like Xu wrote:
> The LBR feature would be enabled on the guest if:
> - the KVM is enabled and the PMU is enabled and,
> - the msr-based-feature IA32_PERF_CAPABILITIES is supporterd and,
> - the supported returned value for lbr_fmt from this msr is not zero.
> 
> The LBR feature would be disabled on the guest if:
> - the msr-based-feature IA32_PERF_CAPABILITIES is unsupporterd OR,
> - qemu set the IA32_PERF_CAPABILITIES msr feature without lbr_fmt values OR,
> - the requested guest vcpu model doesn't support PDCM.
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Richard Henderson <rth@xxxxxxxxxxx>
> Cc: Eduardo Habkost <ehabkost@xxxxxxxxxx>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: Marcel Apfelbaum <marcel.apfelbaum@xxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: qemu-devel@xxxxxxxxxx
> Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> ---
>  hw/i386/pc.c      |  1 +
>  target/i386/cpu.c | 24 ++++++++++++++++++++++--
>  target/i386/cpu.h |  2 ++
>  target/i386/kvm.c |  7 ++++++-
>  4 files changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 3d419d5991..857aff75bb 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -318,6 +318,7 @@ GlobalProperty pc_compat_1_5[] = {
>      { "Nehalem-" TYPE_X86_CPU, "min-level", "2" },
>      { "virtio-net-pci", "any_layout", "off" },
>      { TYPE_X86_CPU, "pmu", "on" },
> +    { TYPE_X86_CPU, "lbr", "on" },

Why is this line here?

>      { "i440FX-pcihost", "short_root_bus", "0" },
>      { "q35-pcihost", "short_root_bus", "0" },
>  };
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 588f32e136..c803994887 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -1142,8 +1142,8 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
>      [FEAT_PERF_CAPABILITIES] = {
>          .type = MSR_FEATURE_WORD,
>          .feat_names = {
> -            NULL, NULL, NULL, NULL,
> -            NULL, NULL, NULL, NULL,
> +            "lbr-fmt-bit-0", "lbr-fmt-bit-1", "lbr-fmt-bit-2", "lbr-fmt-bit-3",
> +            "lbr-fmt-bit-4", "lbr-fmt-bit-5", NULL, NULL,

What about a separate "lbr-fmt" int property instead of
individual bit properties?

What happens if LBR_FMT on the host (returned by
kvm_arch_get_supported_msr_feature(MSR_IA32_PERF_CAPABILITIES) is
different than the one configured for the guest?  Can KVM emulate
a CPU with different LBR_FMT, or it must match the host?

If LBR_FMT must always match the host, the feature needs to block
live migration.  I guess this is already the case because PDCM is
cleared if !cpu->enable_pmu.  Adding PDCM to .unmigratable_flags
is probably a good idea, though.



>              NULL, NULL, NULL, NULL,
>              NULL, "full-width-write", NULL, NULL,
>              NULL, NULL, NULL, NULL,
> @@ -4224,6 +4224,12 @@ static bool lmce_supported(void)
>      return !!(mce_cap & MCG_LMCE_P);
>  }
>  
> +static inline bool lbr_supported(void)
> +{
> +    return kvm_enabled() && (kvm_arch_get_supported_msr_feature(kvm_state,
> +        MSR_IA32_PERF_CAPABILITIES) & PERF_CAP_LBR_FMT);
> +}

You can rewrite this is an accelerator-independent way as:
  (x86_cpu_get_supported_feature_word(FEAT_PERF_CAPABILITIES) & PERF_CAP_LBR_FMT)

However, is this really supposed to return false if LBR_FMT is 000000?

> +
>  #define CPUID_MODEL_ID_SZ 48
>  
>  /**
> @@ -4327,6 +4333,9 @@ static void max_x86_cpu_initfn(Object *obj)
>      }
>  
>      object_property_set_bool(OBJECT(cpu), "pmu", true, &error_abort);
> +    if (lbr_supported()) {
> +        object_property_set_bool(OBJECT(cpu), "lbr", true, &error_abort);

Why is this necessary?

If kvm_arch_get_supported_msr_feature(MSR_IA32_PERF_CAPABILITIES)
return the PERF_CAP_LBR_FMT bits set,
x86_cpu_get_supported_feature_word() will return those bits, and
they will be automatically set at
env->features[FEAT_PERF_CAPABILITIES].

> +    }
>  }
>  
>  static const TypeInfo max_x86_cpu_type_info = {
> @@ -5535,6 +5544,10 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
>          }
>          if (!cpu->enable_pmu) {
>              *ecx &= ~CPUID_EXT_PDCM;
> +            if (cpu->enable_lbr) {
> +                warn_report("LBR is unsupported since guest PMU is disabled.");
> +                exit(1);
> +            }
>          }
>          break;
>      case 2:
> @@ -6553,6 +6566,12 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
>          }
>      }
> +    if (!cpu->max_features && cpu->enable_lbr &&

Why do we need to check for !cpu->max_features here?

> +        !(env->features[FEAT_1_ECX] & CPUID_EXT_PDCM)) {
> +        warn_report("requested vcpu model doesn't support PDCM for LBR.");
> +        exit(1);

Please report errors using error_setg(errp, ...) instead.

> +    }
> +
>      if (cpu->ucode_rev == 0) {
>          /* The default is the same as KVM's.  */
>          if (IS_AMD_CPU(env)) {
> @@ -7187,6 +7206,7 @@ static Property x86_cpu_properties[] = {
>  #endif
>      DEFINE_PROP_INT32("node-id", X86CPU, node_id, CPU_UNSET_NUMA_NODE_ID),
>      DEFINE_PROP_BOOL("pmu", X86CPU, enable_pmu, false),
> +    DEFINE_PROP_BOOL("lbr", X86CPU, enable_lbr, false),

When exactly do we want to set lbr=off explicitly?  What's the
expected outcome when lbr=off?


>  
>      DEFINE_PROP_UINT32("hv-spinlocks", X86CPU, hyperv_spinlock_attempts,
>                         HYPERV_SPINLOCK_NEVER_RETRY),
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index e1a5c174dc..a059913e26 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -357,6 +357,7 @@ typedef enum X86Seg {
>  #define ARCH_CAP_TSX_CTRL_MSR		(1<<7)
>  
>  #define MSR_IA32_PERF_CAPABILITIES      0x345
> +#define PERF_CAP_LBR_FMT      0x3f
>  
>  #define MSR_IA32_TSX_CTRL		0x122
>  #define MSR_IA32_TSCDEADLINE            0x6e0
> @@ -1702,6 +1703,7 @@ struct X86CPU {
>       * capabilities) directly to the guest.
>       */
>      bool enable_pmu;
> +    bool enable_lbr;

This is a good place to document what enable_lbr=true|false
means (see questions above).


>  
>      /* LMCE support can be enabled/disabled via cpu option 'lmce=on/off'. It is
>       * disabled by default to avoid breaking migration between QEMU with
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index b8455c89ed..feb33d5472 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -2690,8 +2690,10 @@ static void kvm_msr_entry_add_perf(X86CPU *cpu, FeatureWordArray f)
>      uint64_t kvm_perf_cap =
>          kvm_arch_get_supported_msr_feature(kvm_state,
>                                             MSR_IA32_PERF_CAPABILITIES);
> -
>      if (kvm_perf_cap) {
> +        if (!cpu->enable_lbr) {
> +            kvm_perf_cap &= ~PERF_CAP_LBR_FMT;
> +        }

Why is this necessary?  If enable_lbr is false,
f[FEAT_PERF_CAPABILITIES] should not have those bits set at all.

>          kvm_msr_entry_add(cpu, MSR_IA32_PERF_CAPABILITIES,
>                          kvm_perf_cap & f[FEAT_PERF_CAPABILITIES]);
>      }
> @@ -2731,6 +2733,9 @@ static void kvm_init_msrs(X86CPU *cpu)
>  
>      if (has_msr_perf_capabs && cpu->enable_pmu) {
>          kvm_msr_entry_add_perf(cpu, env->features);
> +    } else if (!has_msr_perf_capabs && cpu->enable_lbr) {
> +        warn_report("KVM doesn't support MSR_IA32_PERF_CAPABILITIES for LBR.");
> +        exit(1);

This is not the appropriate place to check for unsupported
features.  x86_cpu_realizefn() and/or x86_cpu_filter_features()
is.

>      }
>  
>      if (has_msr_ucode_rev) {
> -- 
> 2.21.3
> 

-- 
Eduardo