linux-kernel - Re: [RFC PATCH 1/8] x86/cpufeatures: Enumerate MOVDIRI instruction

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Message-ID: <20180619213644.GC112652@romley-ivt3.sc.intel.com>
Date:   Tue, 19 Jun 2018 14:36:44 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...hat.com>,
        H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 1/8] x86/cpufeatures: Enumerate MOVDIRI instruction

On Tue, Jun 19, 2018 at 10:57:44AM +0200, Thomas Gleixner wrote:
> On Fri, 15 Jun 2018, Fenghua Yu wrote:
> 
> > MOVDIRI moves doubleword or quadword from register to memory through
> > direct store which is implemented by using write combining (WC) for
> > writing data directly into memory without caching the data.
> 
> And that is useful for what?

Programmable agents can handle streaming offload (e.g. high speed packet
processing in network). Hardware implements a doorbell (tail pointer)
register that is updated by software when adding new work-elements to
the streaming offload work-queue.

MOVDIRI can be used as the doorbell write which is a 4-byte or 8-byte
uncachable write to MMIO. MOVDIRI has lower overhead than other ways
to write the doorbell.

In low latency offload (e.g. Non-Volatile Memory, etc), MOVDIR64B writes
work descriptors (and data in some cases) to device-hosted work-queues
with atomicity.

Thanks.

-Fenghua