linux-kernel - Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu

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Date:   Tue, 14 Nov 2017 22:03:50 -0500
From:   Rik van Riel <riel@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Radim Krcmar <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to
 vcpu_run

On Wed, 2017-11-15 at 08:47 +0800, Wanpeng Li wrote:
> 2017-11-15 5:54 GMT+08:00  <riel@...hat.com>:
> > From: Rik van Riel <riel@...hat.com>
> > 
> > Currently, every time a VCPU is scheduled out, the host kernel will
> > first save the guest FPU/xstate context, then load the qemu
> > userspace
> > FPU context, only to then immediately save the qemu userspace FPU
> > context back to memory. When scheduling in a VCPU, the same
> > extraneous
> > FPU loads and saves are done.
> > 
> > This could be avoided by moving from a model where the guest FPU is
> > loaded and stored with preemption disabled, to a model where the
> > qemu userspace FPU is swapped out for the guest FPU context for
> > the duration of the KVM_RUN ioctl.
> 
> What will happen if CONFIG_PREEMPT is enabled?

The scheduler will save the guest FPU context when a
VCPU thread is preempted, and restore it when it is
scheduled back in.

While inside kvm_arch_vcpu_ioctl_run, the task FPU
context will be the guest FPU context, instead of the
qemu userspace FPU context.