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

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Date:   Wed, 29 May 2019 03:24:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tao Xu <tao3.xu@...el.com>, rkrcmar@...hat.com, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        sean.j.christopherson@...el.com
Cc:     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 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