linux-kernel - Re: [PATCH v6 14/17] powerpc: Add support for setting SPRN

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Message-Id: <1502717435.4493.29.camel@au1.ibm.com>
Date:   Mon, 14 Aug 2017 23:30:35 +1000
From:   Benjamin Herrenschmidt <benh@....ibm.com>
To:     Michael Neuling <mikey@...ling.org>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     stewart@...ux.vnet.ibm.com, linuxppc-dev@...abs.org,
        linux-kernel@...r.kernel.org, apopple@....ibm.com, oohall@...il.com
Subject: Re: [PATCH v6 14/17] powerpc: Add support for setting SPRN_TIDR

On Mon, 2017-08-14 at 17:02 +1000, Michael Neuling wrote:
> > +/*
> > + * We need to assign an unique thread id to each thread in a process. This
> > + * thread id is intended to be used with the Fast Thread-wakeup (aka Core-
> > + * to-core wakeup) mechanism being implemented on top of Virtual Accelerator
> > + * Switchboard (VAS).
> > + *
> > + * To get a unique thread-id per process we could simply use task_pid_nr()
> > + * but the problem is that task_pid_nr() is not yet available for the thread
> > + * when copy_thread() is called. Fixing that would require changing more
> > + * intrusive arch-neutral code in code path in copy_process()?.
> > + *
> > + * Further, to assign unique thread ids within each process, we need an
> > + * atomic field (or an IDR) in task_struct, which again intrudes into the
> > + * arch-neutral code.
> 
> Really?
> 
> > + * So try to assign globally unique thraed ids for now.
> 
> Yuck!

Also CAPI has size limits for the TIDR afaik

Ben.