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Message-ID: <0962c143-2ff9-f157-d258-d16659818e80@redhat.com>
Date:   Wed, 13 Oct 2021 08:26:36 +0200
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Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core
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To:     "Liu, Jing2" <jing2.liu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     "x86@kernel.org" <x86@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        Jing Liu <jing2.liu@linux.intel.com>,
        "seanjc@google.com" <seanjc@google.com>
References: <20211011215813.558681373@linutronix.de>
 <20211011223611.069324121@linutronix.de>
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 <BYAPR11MB3256B39E2A34A09FF64ECC5BA9B79@BYAPR11MB3256.namprd11.prod.outlook.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <BYAPR11MB3256B39E2A34A09FF64ECC5BA9B79@BYAPR11MB3256.namprd11.prod.outlook.com>
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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