Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v2 1/3] KVM: x86: add support for user wait instructions
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     rkrcmar@redhat.com, corbet@lwn.net, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        sean.j.christopherson@intel.com, x86@kernel.org,
        kvm@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, jingqi.liu@intel.com
References: <20190524075637.29496-1-tao3.xu@intel.com>
 <20190524075637.29496-2-tao3.xu@intel.com>
 <419f62f3-69a8-7ec0-5eeb-20bed69925f2@redhat.com>
From:   Tao Xu <tao3.xu@intel.com>
Message-ID: <c1b27714-2eb8-055e-f26c-e17787d83bb6@intel.com>
Date:   Wed, 29 May 2019 10:05:19 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.6.1
MIME-Version: 1.0
In-Reply-To: <419f62f3-69a8-7ec0-5eeb-20bed69925f2@redhat.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 29/05/2019 09:24, Paolo Bonzini wrote:
> On 24/05/19 09:56, Tao Xu wrote:
>> +7.19 KVM_CAP_ENABLE_USR_WAIT_PAUSE
>> +
>> +Architectures: x86
>> +Parameters: args[0] whether feature should be enabled or not
>> +
>> +With this capability enabled, a VM can use UMONITOR, UMWAIT and TPAUSE
>> +instructions. 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). Otherwise, UMONITOR, UMWAIT
>> +and TPAUSE cause an invalid-opcode exception(#UD).
>> +
> 
> There is no need to make it a capability.  You can just check the guest
> CPUID and see if it includes X86_FEATURE_WAITPKG.
> 
> Paolo
> 

Thank you Paolo, but I have another question. I was wondering if it is 
appropriate to enable X86_FEATURE_WAITPKG when QEMU uses "-overcommit 
cpu-pm=on"? Or just enable X86_FEATURE_WAITPKG when QEMU add the feature 
"-cpu host,+waitpkg"? User wait instructions is the wait or pause 
instructions may be executed at any privilege level, but can use 
IA32_UMWAIT_CONTROL to set the maximum time.