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Subject: Re: [PATCH v2 1/3] KVM: x86: add support for user wait instructions
To:     Tao Xu <tao3.xu@intel.com>, rkrcmar@redhat.com, corbet@lwn.net,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        sean.j.christopherson@intel.com
Cc:     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>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <419f62f3-69a8-7ec0-5eeb-20bed69925f2@redhat.com>
Date:   Wed, 29 May 2019 03:24:14 +0200
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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