linux-kernel - Re: [PATCH 0/3]: x86, fpu: unlazy

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Message-ID: <20150130134309.GA7855@redhat.com>
Date:	Fri, 30 Jan 2015 14:43:09 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	Rik van Riel <riel@...hat.com>, Suresh Siddha <sbsiddha@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3]: x86, fpu: unlazy_fpu fixes/cleanups

On 01/30, Oleg Nesterov wrote:
>
> On 01/30, Oleg Nesterov wrote:
> >
> > On 01/29, Dave Hansen wrote:
> > >
> > > On 01/29/2015 01:56 PM, Oleg Nesterov wrote:
> > > > --- x/arch/x86/kernel/traps.c
> > > > +++ x/arch/x86/kernel/traps.c
> > > > @@ -313,7 +313,7 @@ dotraplinkage void do_bounds(struct pt_r
> > > >  	 * It is not directly accessible, though, so we need to
> > > >  	 * do an xsave and then pull it out of the xsave buffer.
> > > >  	 */
> > > > -	fpu_save_init(&tsk->thread.fpu);
> > > > +	unlazy_fpu(tsk);
> > > >  	xsave_buf = &(tsk->thread.fpu.state->xsave);
> > > ...
> > > > 	bndcsr = get_xsave_addr(xsave_buf, XSTATE_BNDCSR);
> > >
> > > Hmm, if the the thread was not using the FPU, and this fails to save
> > > anything in to the xsave_buf, what will bndcsr point to?  It _looks_ to
> > > me like it will just point to uninitialized data since the xsave never
> > > happened.
> > >
> > > Fenghua, shouldn't get_xsave_addr() be checking the xstate bit against
> > > the xsave->xstate_bv?
> >
> > Can't really comment, but let me clarify what I meant just in case...
> >
> > If it was not using FPU then I guess do_bounds() can't happen. However,
> > it can be preempted after conditional_sti(). fpu_save_init() is obviously
> > wrong unless __thread_has_fpu() == T, and this can be false if !eagerfpu
> > or if we add TIF_LOAD_FPU (defer FPU restore until return to userspace).
>
> Forgot to mention... and if we use unlazy_fpu() we should not worry about
> preemption/__thread_has_fpu, we can rely on __save_init_fpu() in __switch_to()
> and/or __thread_has_fpu() check in unlazy_fpu().
>
> But I am afraid I misunderstood your concerns, sorry for noise in this case.

Yes ;)

Perhaps do_bounds() needs the additional

	if (!used_math())
		goto exit_trap;

check?

Oleg.

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