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

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Message-ID: <6b2abf36-2b2f-d721-ae9d-bd2b26785eba@intel.com>
Date:   Wed, 19 Jun 2019 21:32:55 +0800
From:   Tao Xu <tao3.xu@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...ux.intel.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 3:16 PM, Xiaoyao Li wrote:
> 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.
> 

Xiaoyao, you are right. And MSR_IA32_XSS has the same logic. I will fix 
my patch. And we may need to submit a fix patch for MSR_IA32_XSS.