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Date:   Thu, 14 Oct 2021 14:26:31 +0200
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Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core
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To:     Thomas Gleixner <tglx@linutronix.de>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     "x86@kernel.org" <x86@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        Jing Liu <jing2.liu@linux.intel.com>,
        "seanjc@google.com" <seanjc@google.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>
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 <ec9c761d-4b5c-71e2-c1fc-d256b6b78c04@redhat.com> <875ytz7q2u.ffs@tglx>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <875ytz7q2u.ffs@tglx>
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On 14/10/21 14:23, Thomas Gleixner wrote:
>> In principle I don't like it very much; it would be nicer to say "you
>> enable it for QEMU itself via arch_prctl(ARCH_SET_STATE_ENABLE), and for
>> the guests via ioctl(KVM_SET_CPUID2)".  But I can see why you want to
>> keep things simple, so it's not a strong objection at all.
> Errm.
> 
>     qemu()
>       read_config()
>       if (dynamic_features_passthrough())
> 	request_permission(feature)             <- prctl(ARCH_SET_STATE_ENABLE)
> 
>       create_vcpu_threads()
>         ....
> 
>         vcpu_thread()
> 	 kvm_ioctl(ENABLE_DYN_FEATURE, feature) <- KVM ioctl
> 
> That's what I lined out, right?
> 

I meant prctl for QEMU-in-user-mode vs. ioctl QEMU-in-guest-mode (i.e. 
no prctl if only the guest uses it).  But anyway it's just abstract 
"beauty", let's stick to simple. :)

Paolo