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Subject: Re: [PATCH v2] target-i386: add KVM_HINTS_DEDICATED performance hint
To:     Eduardo Habkost <ehabkost@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
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From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <951766c8-3cac-af10-e63d-ea96aa99fa1b@redhat.com>
Date:   Tue, 10 Apr 2018 14:04:03 +0200
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On 10/04/2018 13:57, Eduardo Habkost wrote:
> On Tue, Apr 10, 2018 at 05:13:21PM +0800, Wanpeng Li wrote:
>> Hi Eduardo,
>> 2018-03-09 22:16 GMT+08:00 Eduardo Habkost <ehabkost@redhat.com>:
>>> On Fri, Feb 09, 2018 at 06:15:25AM -0800, Wanpeng Li wrote:
>>>> From: Wanpeng Li <wanpengli@tencent.com>
>>>>
>>>> Add KVM_HINTS_DEDICATED performance hint, guest checks this feature bit
>>>> to determine if they run on dedicated vCPUs, allowing optimizations such
>>>> as usage of qspinlocks.
>>>>
>>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>>>> Cc: Eduardo Habkost <ehabkost@redhat.com>
>>>> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
>>>> ---
>>>> v1 -> v2:
>>>>  * add a new feature word
>>>>
>>>>  target/i386/cpu.c | 14 ++++++++++++++
>>>>  target/i386/cpu.h |  3 +++
>>>>  target/i386/kvm.c |  4 ++++
>>>>  3 files changed, 21 insertions(+)
>>>>
>>>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>>>> index d70954b..e2974ad 100644
>>>> --- a/target/i386/cpu.c
>>>> +++ b/target/i386/cpu.c
>>>> @@ -358,6 +358,20 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
>>>>          .cpuid_eax = KVM_CPUID_FEATURES, .cpuid_reg = R_EAX,
>>>>          .tcg_features = TCG_KVM_FEATURES,
>>>>      },
>>>> +    [FEAT_KVM_HINTS] = {
>>>> +        .feat_names = {
>>>> +            "hint-dedicated", NULL, NULL, NULL,
>>>
>>> I suggest naming it "kvm-hint-dedicated", to indicate it's
>>> KVM-specific.
>>
>> I found this feature is enabled by default w/o kvm-hint-dedicated
>> parameter, any idea how to make it disabled by default?
> 
> You mean on "-cpu host", right?
> 
> The assumption in most of the code is that anything enabled on
> GET_SUPPORTED_CPUID should be enabled on "-cpu host".  This
> shouldn't be the case for FEAT_KVM_HINTS.
> 
> Untested fix below.
> 
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  target/i386/cpu.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 1a6b082b6f..a20fe26573 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -295,6 +295,8 @@ typedef struct FeatureWordInfo {
>      uint32_t tcg_features; /* Feature flags supported by TCG */
>      uint32_t unmigratable_flags; /* Feature flags known to be unmigratable */
>      uint32_t migratable_flags; /* Feature flags known to be migratable */
> +    /* Features that shouldn't be auto-enabled by "-cpu host" */
> +    uint32_t no_autoenable_flags;
>  } FeatureWordInfo;
>  
>  static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
> @@ -400,6 +402,11 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
>          },
>          .cpuid_eax = KVM_CPUID_FEATURES, .cpuid_reg = R_EDX,
>          .tcg_features = TCG_KVM_FEATURES,
> +        /*
> +         * KVM hints aren't auto-enabled by -cpu host, they need to be
> +         * explicitly enabled in the command-line.
> +         */
> +        .no_autoenable_flags = ~0U,
>      },
>      [FEAT_HYPERV_EAX] = {
>          .feat_names = {
> @@ -4062,7 +4069,8 @@ static void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
>               */
>              env->features[w] |=
>                  x86_cpu_get_supported_feature_word(w, cpu->migratable) &
> -                ~env->user_features[w];
> +                ~env->user_features[w] & \
> +                ~feature_word_info[w].no_autoenable_flags;
>          }
>      }
>  
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>