Re: [PATCH v2 1/3] KVM: x86: add support for user wait instructions

Tao Xu
2019-05-28T07:21:56Z

On 28/05/2019 14:11, Wanpeng Li wrote:
> On Tue, 28 May 2019 at 13:16, Tao Xu <tao3.xu@intel.com> wrote:
>>
>>
>> On 27/05/2019 18:30, Peter Zijlstra wrote:
>>> On Fri, May 24, 2019 at 03:56:35PM +0800, Tao Xu wrote:
>>>> This patch adds support for UMONITOR, UMWAIT and TPAUSE instructions
>>>> in kvm, and by default dont't expose it to kvm and provide a capability
>>>> to enable it.
>>>
>>> I'm thinking this should be conditional on the guest being a 1:1 guest,
>>> and I also seem to remember we have bits for that already -- they were
>>> used to disable paravirt spinlocks for example.
>>>
>>
>> Hi Peter,
>>
>> I am wondering if "1:1 guest" means different guests in the same host
>> should have different settings on user wait instructions?
>>
>> User wait instructions(UMONITOR, UMWAIT and TPAUSE) can use in guest
>> only when the VMCS Secondary Processor-Based VM-Execution Control bit 26
>> is 1, otherwise any execution of TPAUSE, UMONITOR, or UMWAIT causes a #UD.
>>
>> So with a capability to enable it, we use qemu kvm_vm_ioctl_enable_cap()
>> to enable it. The qemu link is blew:
>> https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg05810.html
>>
>> By using different QEMU parameters, different guests in the same host
>> would have different features with or without user wait instructions.
>>
>> About "disable paravirt spinlocks" case, I am wondering if it uses
>
> Please refer to a4429e53c9 (KVM: Introduce paravirtualization hints
> and KVM_HINTS_DEDICATED) and b2798ba0b87 (KVM: X86: Choose qspinlock
> when dedicated physical CPUs are available)
Hi Wanpeng,

Thank you! This information really helped me. After I read the code in
KVM/QEMU, I was wondering that with qemu command-line "-cpu
host,+kvm-hint-dedicated", then in KVM,
"kvm_hint_has_feature(KVM_HINTS_DEDICATED)" will be true, am I right?

Tao