Re: [Qemu-devel] [PATCH v4 2/3] kvm: Add support to KVM_GET_MSR_FEATURE_INDEX_LIST and KVM_GET_MSRS system ioctl

[Eduardo Habkost]

Hi,

Thanks for the patch and sorry for taking so long to review it.

On Sun, Sep 02, 2018 at 07:46:06PM +0800, Robert Hoo wrote:
> Add kvm_get_supported_feature_msrs() to get supported MSR feature index list.
> Add kvm_arch_get_supported_msr_feature() to get each MSR features value.
> 
> Signed-off-by: Robert Hoo <address@hidden>
> ---
>  include/sysemu/kvm.h |  2 ++
>  target/i386/cpu.c    |  7 ++---
>  target/i386/kvm.c    | 72 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 78 insertions(+), 3 deletions(-)
> 
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index 0b64b8e..97d8d9d 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -463,6 +463,8 @@ int kvm_vm_check_extension(KVMState *s, unsigned int 
> extension);
>  
>  uint32_t kvm_arch_get_supported_cpuid(KVMState *env, uint32_t function,
>                                        uint32_t index, int reg);
> +uint32_t kvm_arch_get_supported_msr_feature(KVMState *s, uint32_t index);
> +
>  
>  void kvm_set_sigmask_len(KVMState *s, unsigned int sigmask_len);
>  
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index a252c26..0160e97 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -3670,7 +3670,7 @@ static uint32_t 
> x86_cpu_get_supported_feature_word(FeatureWord w,
>                                                     bool migratable_only)
>  {
>      FeatureWordInfo *wi = &feature_word_info[w];
> -    uint32_t r;
> +    uint32_t r = 0;
>  
>      if (kvm_enabled()) {
>          switch (wi->type) {
> @@ -3679,8 +3679,9 @@ static uint32_t 
> x86_cpu_get_supported_feature_word(FeatureWord w,
>                                                  wi->cpuid.ecx,
>                                                  wi->cpuid.reg);
>              break;
> -        default:
> -            r = 0;
> +        case MSR_FEATURE_WORD:
> +            r = kvm_arch_get_supported_msr_feature(kvm_state,
> +                        wi->msr.index);

If you move this patch before patch 1/3, this hunk could be part
of patch 1/3.

>              break;
>          }
>      } else if (hvf_enabled()) {
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index 0b2a07d..bfd8088 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -107,6 +107,7 @@ static int has_pit_state2;
>  static bool has_msr_mcg_ext_ctl;
>  
>  static struct kvm_cpuid2 *cpuid_cache;
> +static struct kvm_msr_list *kvm_feature_msrs;
>  
>  int kvm_has_pit_state2(void)
>  {
> @@ -420,6 +421,33 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, 
> uint32_t function,
>      return ret;
>  }
>  
> +uint32_t kvm_arch_get_supported_msr_feature(KVMState *s, uint32_t index)
> +{
> +    struct {
> +        struct kvm_msrs info;
> +        struct kvm_msr_entry entries[1];
> +    } msr_data;
> +    uint32_t ret;
> +
> +    if (kvm_feature_msrs == NULL) { /*ARCH doesn't support feature MSRs*/

Nit: normally comments have spaces after "/*" and before "*/".

Also: what do you mean by "ARCH"?  Do you mean "host kernel"?


> +        return 0;
> +    }
> +
> +    msr_data.info.nmsrs = 1;
> +    msr_data.entries[0].index = index;
> +
> +    ret = kvm_ioctl(s, KVM_GET_MSRS, &msr_data);
> +
> +    if (ret != 1) {

If the MSR is not supported by the host kernel, it must not be a
fatal error.  We should just return 0 on that case.

Probably the best way to ensure that is to check if the MSR is
listed on kvm_feature_msrs before calling KVM_GET_MSRS (and
return 0 if the MSR is not on the list).


> +        fprintf(stderr, "KVM get MSR (index=0x%x) feature failed, %s\n",
> +            index, strerror(-ret));

Please use error_report() instead of fprintf(stderr).

> +        exit(1);

I'm unsure if exit(1) is the best option here, but at least this
is consistent with error handling kvm_arch_get_supported_cpuid().


> +    }
> +
> +    return msr_data.entries[0].data;
> +}
> +
> +
>  typedef struct HWPoisonPage {
>      ram_addr_t ram_addr;
>      QLIST_ENTRY(HWPoisonPage) list;
> @@ -1239,6 +1267,45 @@ void kvm_arch_do_init_vcpu(X86CPU *cpu)
>      }
>  }
>  
> +static int kvm_get_supported_feature_msrs(KVMState *s)
> +{
> +    int ret = 0;
> +
> +    if (kvm_feature_msrs != NULL) {
> +        return 0;
> +    }
> +
> +    if (!kvm_check_extension(s, KVM_CAP_GET_MSR_FEATURES)) {
> +        return -1;

There's nothing wrong with not supporting
KVM_CAP_GET_MSR_FEATURES.  Why not return 0?


> +    }
> +
> +    struct kvm_msr_list msr_list;
> +
> +    msr_list.nmsrs = 0;
> +    ret = kvm_ioctl(s, KVM_GET_MSR_FEATURE_INDEX_LIST, &msr_list);
> +    if (ret < 0 && ret != -E2BIG) {

You print an error to stderr if (ret < 0) below, but don't print
anything here.  Seems inconsistent.

> +        return ret;
> +    }
> +
> +    assert(msr_list.nmsrs > 0);
> +    kvm_feature_msrs = (struct kvm_msr_list *) \
> +        g_malloc0(sizeof(msr_list) +
> +                 msr_list.nmsrs * sizeof(msr_list.indices[0]));
> +
> +    kvm_feature_msrs->nmsrs = msr_list.nmsrs;
> +    ret = kvm_ioctl(s, KVM_GET_MSR_FEATURE_INDEX_LIST, kvm_feature_msrs);

kvm_arch_get_supported_msr_feature() is only checking if
kvm_feature_msrs is NULL, and nothing else.

What exactly is the point of calling
KVM_GET_MSR_FEATURE_INDEX_LIST and saving data at
kvm_feature_msrs, if no other code is ever looking at the
returned data?


> +
> +    if (ret < 0) {
> +        fprintf(stderr, "Fetch KVM feature MSRs failed: %s\n",
> +            strerror(-ret));

Please use error_report() instead of fprintf(stderr).

> +        g_free(kvm_feature_msrs);
> +        kvm_feature_msrs = NULL;
> +        return ret;
> +    }
> +
> +    return 0;
> +}
> +
>  static int kvm_get_supported_msrs(KVMState *s)
>  {
>      static int kvm_supported_msrs;
> @@ -1392,6 +1459,11 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>          return ret;
>      }
>  
> +    ret = kvm_get_supported_feature_msrs(s);
> +    if (ret < 0) { /*if MSR based features aren't supported, ignore it.*/
> +        warn_report("Get supported feature MSRs failed.");

We must not print a warning only because KVM_CAP_GET_MSR_FEATURES
isn't supported by the host kernel.

If KVM_GET_MSR_FEATURE_INDEX_LIST fails, on the other hand, we
probably should make it a fatal error and not a warning.

> +    }
> +
>      uname(&utsname);
>      lm_capable_kernel = strcmp(utsname.machine, "x86_64") == 0;
>  
> -- 
> 1.8.3.1
> 
> 

-- 
Eduardo