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

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Message-ID: <689cf4f5-6004-de51-f5ca-9a7acee37499@redhat.com>
Date:   Fri, 15 Oct 2021 17:53:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Liu, Jing2" <jing2.liu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>
Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

On 15/10/21 16:24, Liu, Jing2 wrote:
>> fpu_swap_kvm_fpu(bool enter_guest, u64 guest_needs_features) {
>>          possibly_reallocate(enter_guest, guest_needs_features);
> When KVM traps guest wrmsr XFD in #NM, I think KVM need allocate
> fpstate buffer for full features.

You mean XCR0 and XFD (not XFD in #NM), but yeah at the point of 
fpu_swap_kvm_fpu we are in atomic context.

Still, for now the first pass of AMX implementation doesn't need to do 
anything but swap the pointers, and it can simply allocate the full 
buffer at vCPU creation.

Paolo

> Because in next vmexit, guest might have dynamic state and KVM
> can be preempted before running fpu_swap_kvm_fpu().
> Thus, here the current->thread.fpu.fpstate already has enough space
> for saving guest.