linux-kernel - Re: [PATCH 2/2] i387: split up <asm/i387.h> into exported and internal interfaces

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Message-ID: <4F4D25A2.70307@redhat.com>
Date:	Tue, 28 Feb 2012 21:06:10 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>, Josh Boyer <jwboyer@...il.com>,
	Jongman Heo <jongman.heo@...sung.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/2] i387: split up <asm/i387.h> into exported and internal
 interfaces

On 02/28/2012 08:34 PM, Linus Torvalds wrote:
> On Tue, Feb 28, 2012 at 10:09 AM, Avi Kivity <avi@...hat.com> wrote:
> >
> > This is done by preempt notifiers.  Whenever a task switch happens we
> > push the guest fpu state into memory (if loaded) and let the normal
> > stuff happen.  So the if we had a task switch during instruction
> > emulation, for example, then we'd get the "glacial and stupid path" to fire.
>
> Oh christ.
>
> This is exactly what the scheduler has ALWAYS ALREADY DONE FOR YOU.

No, the scheduler saves the state into task_struct.  I need it saved
into the vcpu structure.  We have two fpu states, the user state, and
the guest state.  APIs that take a task_struct as a parameter, or
reference current implicitly, aren't going to work.

> That's what the i387 save-and-restore code is all about. What's the
> advantage of just re-implementing it in non-obvious ways?
>
> Stop doing it. You get *zero* advantages from just doing what the
> scheduler natively does for you, and the scheduler does it *better*.

The scheduler does something different.

What I'd ideally want is

  struct fpu {
      int cpu;  /* -1 = not loaded */
      union thread_xstate *state;
  };

Perhaps with a struct fpu_ops *ops if needed.  We could then let various
users' fpus float around freely and only save/load them at the last moment.


-- 
error compiling committee.c: too many arguments to function

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