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Subject: Re: [PATCH] x86,kvm: move qemu/guest FPU switching out to vcpu_run
To:     Rik van Riel <riel@redhat.com>,
        David Hildenbrand <david@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, rkrcmar@redhat.com, borntraeger@de.ibm.com
References: <20171114001223.441ea2ca@annuminas.surriel.com>
 <a3c862c2-e5f9-f394-885c-10fde7904f03@redhat.com>
 <1510682877.30057.1.camel@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <d5beb041-9a21-84b3-9b72-a8aa7cfb0e2e@redhat.com>
Date:   Tue, 14 Nov 2017 19:09:17 +0100
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From: Paolo Bonzini <pbonzini@redhat.com>
To: Rik van Riel <riel@redhat.com>, David Hildenbrand <david@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de,
 rkrcmar@redhat.com, borntraeger@de.ibm.com
Message-ID: <d5beb041-9a21-84b3-9b72-a8aa7cfb0e2e@redhat.com>
Subject: Re: [PATCH] x86,kvm: move qemu/guest FPU switching out to vcpu_run
References: <20171114001223.441ea2ca@annuminas.surriel.com>
 <a3c862c2-e5f9-f394-885c-10fde7904f03@redhat.com>
 <1510682877.30057.1.camel@redhat.com>
In-Reply-To: <1510682877.30057.1.camel@redhat.com>

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On 14/11/2017 19:07, Rik van Riel wrote:
> My question is, can the in-kernel emulator code ever
> be called from OUTSIDE the KVM_RUN ioctl loop?

No, it can't.  This makes the patch much more appealing...

Paolo

> If so, we need to restore the user FPU context before
> returning from the emulator code. Given that the current
> emulator code does not do that, I suspect this is not
> the case. I also see no path from the kvm ioctl into
> the emulator code, other than via KVM_RUN.
>=20
> The FPU and XSAVE ioctls all work on the saved
> vcpu->arch.guest_fpu data, and never directly on the
> registers.


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