linux-kernel - Re: [PATCH] x86,kvm: move qemu/guest FPU switching out to vcpu

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Date:   Tue, 14 Nov 2017 19:09:17 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Rik van Riel <riel@...hat.com>,
        David Hildenbrand <david@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, rkrcmar@...hat.com, borntraeger@...ibm.com
Subject: Re: [PATCH] x86,kvm: move qemu/guest FPU switching out to vcpu_run

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.
> 
> 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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