linux-kernel - Re: [PATCH v8 0/3] KVM: x86: Enable user wait instructions

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Date:   Fri, 19 Jul 2019 19:18:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tao Xu <tao3.xu@...el.com>, rkrcmar@...hat.com,
        sean.j.christopherson@...el.com, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        fenghua.yu@...el.com, xiaoyao.li@...ux.intel.com,
        jingqi.liu@...el.com
Subject: Re: [PATCH v8 0/3] KVM: x86: Enable user wait instructions

On 19/07/19 08:31, Tao Xu wrote:
> Ping for comments :)

Hi, I'll look at it for 5.4, right after the merge window.

Paolo

> On 7/16/2019 2:55 PM, Tao Xu wrote:
>> UMONITOR, UMWAIT and TPAUSE are a set of user wait instructions.
>>
>> UMONITOR arms address monitoring hardware using an address. A store
>> to an address within the specified address range triggers the
>> monitoring hardware to wake up the processor waiting in umwait.
>>
>> UMWAIT instructs the processor to enter an implementation-dependent
>> optimized state while monitoring a range of addresses. The optimized
>> state may be either a light-weight power/performance optimized state
>> (c0.1 state) or an improved power/performance optimized state
>> (c0.2 state).
>>
>> TPAUSE instructs the processor to enter an implementation-dependent
>> optimized state c0.1 or c0.2 state and wake up when time-stamp counter
>> reaches specified timeout.
>>
>> Availability of the user wait instructions is indicated by the presence
>> of the CPUID feature flag WAITPKG CPUID.0x07.0x0:ECX[5].
>>
>> The patches enable the umonitor, umwait and tpause features in KVM.
>> 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. 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 (the time to delay
>> relative to the VM’s timestamp counter).
>>
>> The release document ref below link:
>> Intel 64 and IA-32 Architectures Software Developer's Manual,
>> https://software.intel.com/sites/default/files/\
>> managed/39/c5/325462-sdm-vol-1-2abcd-3abcd.pdf
>>
>> Changelog:
>> v8:
>>     Add vmx_waitpkg_supported() helper (Sean)
>>     Add an accessor to expose umwait_control_cached (Sean)
>>     Set msr_ia32_umwait_control in vcpu_vmx u32 and raise #GP when
>>     [63:32] is set when rdmsr. (Sean)
>>     Introduce a common exit helper handle_unexpected_vmexit (Sean)
>> v7:
>>     Add nested support for user wait instructions (Paolo)
>>     Use the test on vmx->secondary_exec_control to replace
>>     guest_cpuid_has (Paolo)
>> v6:
>>     add check msr_info->host_initiated in get/set msr(Xiaoyao)
>>     restore the atomic_switch_umwait_control_msr()(Xiaoyao)
>>
>> Tao Xu (3):
>>    KVM: x86: Add support for user wait instructions
>>    KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL
>>    KVM: vmx: Introduce handle_unexpected_vmexit and handle WAITPKG vmexit
>>
>>   arch/x86/include/asm/vmx.h      |  1 +
>>   arch/x86/include/uapi/asm/vmx.h |  6 ++-
>>   arch/x86/kernel/cpu/umwait.c    |  6 +++
>>   arch/x86/kvm/cpuid.c            |  2 +-
>>   arch/x86/kvm/vmx/capabilities.h |  6 +++
>>   arch/x86/kvm/vmx/nested.c       |  5 ++
>>   arch/x86/kvm/vmx/vmx.c          | 83 ++++++++++++++++++++++++++-------
>>   arch/x86/kvm/vmx/vmx.h          |  9 ++++
>>   arch/x86/kvm/x86.c              |  1 +
>>   9 files changed, 101 insertions(+), 18 deletions(-)
>>
>