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Date: Thu, 15 Feb 2007 19:15:24 -0800 (PST) From: Davide Libenzi <davidel@...ilserver.org> To: Ingo Molnar <mingo@...e.hu> cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Arjan van de Ven <arjan@...radead.org>, Christoph Hellwig <hch@...radead.org>, Andrew Morton <akpm@....com.au>, Alan Cox <alan@...rguk.ukuu.org.uk>, Ulrich Drepper <drepper@...hat.com>, Zach Brown <zach.brown@...cle.com>, Evgeniy Polyakov <johnpol@....mipt.ru>, "David S. Miller" <davem@...emloft.net>, Suparna Bhattacharya <suparna@...ibm.com>, Thomas Gleixner <tglx@...utronix.de> Subject: Re: [patch 09/14] syslets: x86, add move_user_context() method On Thu, 15 Feb 2007, Davide Libenzi wrote: > On Thu, 15 Feb 2007, Ingo Molnar wrote: > > > /* > > + * Move user-space context from one kernel thread to another. > > + * This includes registers and FPU state. Callers must make > > + * sure that neither task is running user context at the moment: > > + */ > > +void > > +move_user_context(struct task_struct *new_task, struct task_struct *old_task) > > +{ > > + struct pt_regs *old_regs = task_pt_regs(old_task); > > + struct pt_regs *new_regs = task_pt_regs(new_task); > > + union i387_union *tmp; > > + > > + *new_regs = *old_regs; > > + /* > > + * Flip around the FPU state too: > > + */ > > + tmp = new_task->thread.i387; > > + new_task->thread.i387 = old_task->thread.i387; > > + old_task->thread.i387 = tmp; > > +} > > Let's say that old_task ("prev" at the incoming schedule) has TS_USEDFPU > set. Its context gets moved to the new_task (the one returning to > userspace) *before* the __unlazy_fpu() done in __switch_to(). The > __unlazy_fpu() at the following schedule will save the state to the old > new_task context, and that fine as far as the going-to-sleep task goes. > The next fault happening in new_task (return to userspace one) will reload > a non up2date context (the one we got from old_task, but never hit by > the __unlazy_fpu() flush). Right? Yeah. Given TS_USEDFPU set, before move_user_context(): CPU => FPUc NTSK => FPUn OTSK => FPUo After move_user_context(): CPU => FPUc NTSK => FPUo OTSK => FPUn After the incoming __unlazy_fpu() in __switch_to(): CPU => FPUc NTSK => FPUo OTSK => FPUc After the first fault in NTSK: CPU => FPUo NTSK => FPUo OTSK => FPUc So NTSK loads a non up2date FPUo, instead of the FPUc that was the "dirty" context to migrate (since TS_USEDFPU was set). I think you need an early __unlazy_fpu() in that case. - Davide - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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