linux-kernel - Re: [RFC PATCH v3 for 4.15 08/24] Provide cpu

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Date:   Mon, 20 Nov 2017 19:03:00 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v3 for 4.15 08/24] Provide cpu_opv system call

On Mon, 20 Nov 2017, Thomas Gleixner wrote:
> On Mon, 20 Nov 2017, Mathieu Desnoyers wrote:
> > >> + * The reason why we require all pointer offsets to be calculated by
> > >> + * user-space beforehand is because we need to use get_user_pages_fast()
> > >> + * to first pin all pages touched by each operation. This takes care of
> > > 
> > > That doesnt explain it either.
> > 
> > What kind of explication are you looking for here ? Perhaps being too close
> > to the implementation prevents me from understanding what is unclear from
> > your perspective.
> 
> What the heck are pointer offsets?
> 
> The ops have one or two pointer(s) to a lump of memory. So if a pointer
> points to the wrong lump of memory then you're screwed, but that's true for
> all pointers handed to the kernel.

So I think you mix here the 'user space programmer guide - how to set up
the magic ops - into the kernel side documentation. The kernel side does
not care about the pointers, as long as they are valid to access.

Again. Try to explain things at the conceptual level.

Thanks,

	tglx