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

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Date:   Wed, 13 Oct 2021 17:05:44 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 13/10/21 16:59, Andy Lutomirski wrote:
>> 
>> Thinking more about it, #NM only has to be trapped if XCR0 enables
>> a dynamic feature.  In other words, the guest value of XFD can be
>> limited to (host_XFD|guest_XFD) & guest_XCR0.  This avoids that
>> KVM unnecessarily traps for old guests that use CR0.TS.
>> 
> You could simplify this by allocating the state the first time XCR0
> enables the feature in question.
> 
> (This is how regular non-virt userspace*should*  work too, but it
> looks like I’ve probably been outvoted on that front…)

Good point, you could do that too and do the work on the XCR0 vmexit 
instead of #NM.

Paolo