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

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Date:   Wed, 13 Oct 2021 08:26:36 +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>
Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

On 13/10/21 08:15, Liu, Jing2 wrote:
> After KVM passthrough XFD to guest, when vmexit opening
> irq window and KVM is interrupted, kernel softirq path can call
> kernel_fpu_begin() to touch xsave state. This function does
> XSAVES. If guest XFD[18] is 1, and with guest AMX state in register,
> then guest AMX state is lost by XSAVES.

Yes, the host value of XFD (which is zero) has to be restored after 
vmexit.  See how KVM already handles SPEC_CTRL.

Passthrough of XFD is only enabled after the guest has caused an #NM 
vmexit and the full XSAVE state has been dynamically allocated, 
therefore it is always possible to do an XSAVES even from atomic context.

Paolo