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 16:16:42 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Rik van Riel <riel@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        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

* Rik van Riel <riel@...hat.com> wrote:

> 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?

So the vCPU threads have host side FPU (user-space) state - whatever FPU state 
Qemu has?

One solution to that overhead, without complicating the FPU state machine in any 
way, would be to add a facility to drop/reacquire that FPU state.

That should automatically result in zero FPU state switching AFAICS: kernel 
threads don't do FPU state switching either.

The vCPU threads sometimes do return to user-space, when they get some deep 
exception that needs to be handled by Qemu, right? This aspect shouldn't be a big 
problem either, because the regular calling convention is to call (synchronous) 
system calls without holding FPU state, right?

I.e. the vCPU /dev/kvm ioctl() could drop/re-map the FPU state with very little 
overhead (i.e. no full save/restore required in that code path either), when it 
enters/exits vCPU mode.

Thanks,

	Ingo