netdev - Re: Kernel oops with 2.6.26, padlock and ipsec: probably problem with fpu state changes

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Date:	Mon, 11 Aug 2008 13:19:01 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Wolfgang Walter <wolfgang.walter@...m.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"viro@...IV.linux.org.uk" <viro@...iv.linux.org.uk>,
	"vegard.nossum@...il.com" <vegard.nossum@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Kernel oops with 2.6.26, padlock and ipsec: probably problem with fpu state changes

On Mon, Aug 11, 2008 at 12:24:59PM -0700, H. Peter Anvin wrote:
> Ingo Molnar wrote:
> > * Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> >
> >> Reported-and-bisected-by: Wolfgang Walter <wolfgang.walter@...m.de>
> >> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
> >
> > no fundamental objection to the x86 bits.
> >
> > shouldnt this:
> >
> > +       if (!in_interrupt())
> > +               return 0;
> >
> > just be eliminated and the cr0/TS save/restore be made unconditional?
> > irq-assymetric APIs are not nice in general.
> >
> > Reading/setting cr0 isnt _that_ slow. (or if it is, by how much does it
> > slow things down, exactly?)
> >
> 
> Setting it is relatively slow.  I think that's part of the reason for
> special instructions to muck with the TS flag.
> 
> Reading it might be slow on obsolete processors.

In addition to the slowness(Wolf has collected some data with earlier
patch and I think it showed a double digit increase in cpu utilization with
some crypt tests):

we can't unconditionally do clts() in the process context. We have
to disable pre-emption to avoid interactions with context switch and
lazy restore. So there will be RT latency issues aswell.

thanks,
suresh
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