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

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Date:   Wed, 13 Oct 2021 16:14:23 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     the arch/x86 maintainers <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 list <kvm@...r.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

On Wed, Oct 13 2021 at 14:26, Paolo Bonzini wrote:

> On 13/10/21 12:14, Andy Lutomirski wrote:
>>> I think it's simpler to always wait for #NM, it will only happen
>>> once per vCPU.  In other words, even if the guest clears XFD before
>>> it generates #NM, the guest_fpu's XFD remains nonzero and an #NM
>>> vmexit is possible.  After #NM the guest_fpu's XFD is zero; then
>>> passthrough can happen and the #NM vmexit trap can be disabled.
>>
>> This will stop being at all optimal when Intel inevitably adds
>> another feature that uses XFD.  In the potentially infinite window in
>> which the guest manages XFD and #NM on behalf of its userspace and
>> when the guest allocates the other hypothetical feature, all the #NMs
>> will have to be trapped by KVM.
>
> The reason is that it's quite common to simply let the guest see all 
> CPUID bits that KVM knows about.

On fleets the cpu features exposed to guests matter a lot to ensure
migratability and I would be surprised when such a feature would just
be universally available to anyone.

Thanks,

        tglx