linux-kernel - Re: Thoughts of AMX KVM support based on latest kernel

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Date:   Tue, 16 Nov 2021 16:05:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Liu, Jing2" <jing2.liu@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>
Subject: Re: Thoughts of AMX KVM support based on latest kernel

On Tue, Nov 16, 2021, Thomas Gleixner wrote:
> > One of potential drawbacks of the Option 2 might be additional 
> > checks in the host, although we can minimize the impact by having
> > CONFIG_KVM_TBD. We believe that the case
> > "XFD != 0 and XINUSE != 0" should be very infrequent.
> 
> I really don't like the idea of having an extra check in switch_to().
> 
> Can we start simple and do something like the uncompiled below and see
> how much overhead it creates?
> 
> Thanks,
> 
>         tglx
> ---

...

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2686f2edb47c..9425fdbb4806 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9576,6 +9576,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	vcpu->arch.last_vmentry_cpu = vcpu->cpu;
>  	vcpu->arch.last_guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>  
> +	kvm_update_guest_xfd_state();

Is there a reason the XFD switch can't key off TIF_NEED_FPU_LOAD a la the other
FPU stuff?  I.e. piggyback this snippet in vcpu_enter_guest():

	if (test_thread_flag(TIF_NEED_FPU_LOAD))
		switch_fpu_return();
> +
>  	vcpu->mode = OUTSIDE_GUEST_MODE;
>  	smp_wmb();
>  
> 
>