linux-kernel - Re: [PATCH v4 1/3] KVM: x86: add support for user wait instructions

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Date:   Wed, 19 Jun 2019 15:16:52 +0800
From:   Xiaoyao Li <xiaoyao.li@...ux.intel.com>
To:     Tao Xu <tao3.xu@...el.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com, corbet@....net, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        sean.j.christopherson@...el.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        fenghua.yu@...el.com, jingqi.liu@...el.com
Subject: Re: [PATCH v4 1/3] KVM: x86: add support for user wait instructions

On 6/19/2019 3:01 PM, Tao Xu wrote:
> On 6/19/2019 2:23 PM, Xiaoyao Li wrote:
>>
>>
>> On 6/19/2019 2:09 PM, Tao Xu wrote:
>>> UMONITOR, UMWAIT and TPAUSE are a set of user wait instructions.
>>> This patch adds support for user wait instructions in KVM. Availability
>>> of the user wait instructions is indicated by the presence of the CPUID
>>> feature flag WAITPKG CPUID.0x07.0x0:ECX[5]. User wait instructions may
>>> be executed at any privilege level, and use IA32_UMWAIT_CONTROL MSR to
>>> set the maximum time.
>>>
>>> The behavior of user wait instructions in VMX non-root operation is
>>> determined first by the setting of the "enable user wait and pause"
>>> secondary processor-based VM-execution control bit 26.
>>>     If the VM-execution control is 0, UMONITOR/UMWAIT/TPAUSE cause
>>> an invalid-opcode exception (#UD).
>>>     If the VM-execution control is 1, treatment is based on the
>>> setting of the “RDTSC exiting” VM-execution control. Because KVM never
>>> enables RDTSC exiting, if the instruction causes a delay, the amount of
>>> time delayed is called here the physical delay. The physical delay is
>>> first computed by determining the virtual delay. If
>>> IA32_UMWAIT_CONTROL[31:2] is zero, the virtual delay is the value in
>>> EDX:EAX minus the value that RDTSC would return; if
>>> IA32_UMWAIT_CONTROL[31:2] is not zero, the virtual delay is the minimum
>>> of that difference and AND(IA32_UMWAIT_CONTROL,FFFFFFFCH).
>>>
>>> Because umwait and tpause can put a (psysical) CPU into a power saving
>>> state, by default we dont't expose it to kvm and enable it only when
>>> guest CPUID has it.
>>>
>>> Detailed information about user wait instructions can be found in the
>>> latest Intel 64 and IA-32 Architectures Software Developer's Manual.
>>>
>>> Co-developed-by: Jingqi Liu <jingqi.liu@...el.com>
>>> Signed-off-by: Jingqi Liu <jingqi.liu@...el.com>
>>> Signed-off-by: Tao Xu <tao3.xu@...el.com>
>>> ---
>>>
>>> no changes in v4.
>>> ---
>>>   arch/x86/include/asm/vmx.h      | 1 +
>>>   arch/x86/kvm/cpuid.c            | 2 +-
>>>   arch/x86/kvm/vmx/capabilities.h | 6 ++++++
>>>   arch/x86/kvm/vmx/vmx.c          | 4 ++++
>>>   4 files changed, 12 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
>>> index a39136b0d509..8f00882664d3 100644
>>> --- a/arch/x86/include/asm/vmx.h
>>> +++ b/arch/x86/include/asm/vmx.h
>>> @@ -69,6 +69,7 @@
>>>   #define SECONDARY_EXEC_PT_USE_GPA        0x01000000
>>>   #define SECONDARY_EXEC_MODE_BASED_EPT_EXEC    0x00400000
>>>   #define SECONDARY_EXEC_TSC_SCALING              0x02000000
>>> +#define SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE    0x04000000
>>>   #define PIN_BASED_EXT_INTR_MASK                 0x00000001
>>>   #define PIN_BASED_NMI_EXITING                   0x00000008
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index e18a9f9f65b5..48bd851a6ae5 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -405,7 +405,7 @@ static inline int __do_cpuid_ent(struct 
>>> kvm_cpuid_entry2 *entry, u32 function,
>>>           F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ |
>>>           F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
>>>           F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
>>> -        F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B);
>>> +        F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/;
>>>       /* cpuid 7.0.edx*/
>>>       const u32 kvm_cpuid_7_0_edx_x86_features =
>>> diff --git a/arch/x86/kvm/vmx/capabilities.h 
>>> b/arch/x86/kvm/vmx/capabilities.h
>>> index d6664ee3d127..fd77e17651b4 100644
>>> --- a/arch/x86/kvm/vmx/capabilities.h
>>> +++ b/arch/x86/kvm/vmx/capabilities.h
>>> @@ -253,6 +253,12 @@ static inline bool cpu_has_vmx_tsc_scaling(void)
>>>           SECONDARY_EXEC_TSC_SCALING;
>>>   }
>>> +static inline bool vmx_waitpkg_supported(void)
>>> +{
>>> +    return vmcs_config.cpu_based_2nd_exec_ctrl &
>>> +        SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE;
>>
>> Shouldn't it be
>>      return vmx->secondary_exec_control &
>>                  SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE;   ?
> 
> vmx->secondary_exec_control is another way to get SECONDARY_EXEC_CONTROL 
> but it need to add vmx as input. Use vmcs_config.cpu_based_2nd_exec_ctrl 
>   is easy expand to use in other place.
> 

This patch want to enable waitpkg for guest only when 
guest_cpuid_has(vcpu, X86_FEATURE_WAITPKG), and it only updates 
vmx->secondary_exec_control based on guest's cpuid when setup vcpu. But 
vmcs_config remains unchanged.  So using vmcs_config there is wrong.