linux-kernel - Re: [patch 3/4] x86/kvm: Convert FPU handling to a single swap buffer

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Message-ID: <b1c49069-437c-7aa6-531d-6651dad72015@redhat.com>
Date:   Mon, 18 Oct 2021 13:55:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Liu, Jing2" <jing2.liu@...el.com>,
        "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, "Nakajima, Jun" <jun.nakajima@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [patch 3/4] x86/kvm: Convert FPU handling to a single swap buffer

On 17/10/21 19:03, Thomas Gleixner wrote:
>   	 */
> -	fpu_swap_kvm_fpu(vcpu->arch.user_fpu, vcpu->arch.guest_fpu,
> -			 ~XFEATURE_MASK_PKRU);
> +	fpu_swap_kvm_fpstate(&vcpu->arch.guest_fpu, true, ~XFEATURE_MASK_PKRU);
>   	trace_kvm_fpu(1);
>   }
>   
>   /* When vcpu_run ends, restore user space FPU context. */
>   static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>   {
> -	/*
> -	 * Guests with protected state have guest_fpu == NULL which makes
> -	 * swap only restore the host state.
> -	 */
> -	fpu_swap_kvm_fpu(vcpu->arch.guest_fpu, vcpu->arch.user_fpu, ~0ULL);
> +	fpu_swap_kvm_fpstate(&vcpu->arch.guest_fpu, false, ~0ULL);

The restore mask can be ~XFEATURE_MASK_PKRU in this case tool this way 
it's constant and you can drop the third argument to the function.

Also perhaps it could be useful to add an

if (WARN_ON_ONCE(cur_fps->is_guest == enter_guest))
	return;

at the top of fpu_swap_kvm_fpstate, since the is_guest member (at least 
for now?) is only used for such kind of assertion.

Paolo