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

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Date:   Wed, 29 May 2019 04:38:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tao Xu <tao3.xu@...el.com>
Cc:     rkrcmar@...hat.com, corbet@....net, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        sean.j.christopherson@...el.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, jingqi.liu@...el.com
Subject: Re: [PATCH v2 1/3] KVM: x86: add support for user wait instructions

On 29/05/19 04:05, Tao Xu wrote:
>>
> 
> 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"?

"-overcommit" only establishes the behavior of KVM, it doesn't change
the cpuid bits.  So you'd need "-cpu" as well.

Paolo

> 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.