linux-kernel - Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

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Message-ID: <0a5aa9d3-e0d4-266e-5e25-021a5ea9c611@redhat.com>
Date:   Thu, 14 Oct 2021 14:26:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

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