linux-kernel - Re: [PATCH 5/7] x86/fpu: Change fpu->fpregs_active users to fpu->fpstate

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Date:   Thu, 26 Jan 2017 09:44:12 -0500
From:   Rik van Riel <riel@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 5/7] x86/fpu: Change fpu->fpregs_active users to
 fpu->fpstate_active

On Thu, 2017-01-26 at 12:26 +0100, Ingo Molnar wrote:
> We want to simplify the FPU state machine by eliminating fpu-
> >fpregs_active,
> and we can do that because the two state flags (::fpregs_active and
> ::fpstate_active) are set essentially together.
> 
> The old lazy FPU switching code used to make a distinction - but
> there's
> no lazy switching code anymore, we always switch in an 'eager'
> fashion.

I've been working for a while now to fix that for
KVM VCPU threads.

Currently when we switch to a VCPU thread, we first
load that thread's userspace FPU context, and then
soon after we save that, and load the guest side FPU
context.

When a VCPU thread goes idle, we also go through
two FPU context transitions.

In order to skip the unnecessary FPU context switches
for VCPU threads, I have been relying on separate
fpstate_active and fpregs_active states.

Do you have any ideas on how I could implement that
kind of change without separate fpstate_active and
fpregs_active states?