linux-kernel - Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

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Message-ID: <20180321122807.GB3245@chelsio.com>
Date:   Wed, 21 Mar 2018 17:58:08 +0530
From:   Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Ganesh GR <ganeshgr@...lsio.com>,
        Nirranjan Kirubaharan <nirranjan@...lsio.com>,
        Indranil Choudhury <indranil@...lsio.com>
Subject: Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

On Tuesday, March 03/20/18, 2018 at 20:10:19 +0530, David Laight wrote:
> From: Rahul Lakkireddy
> > Sent: 20 March 2018 13:32
> ...
> > On High Availability Server, the logs of the failing system must be
> > collected as quickly as possible.  So, we're concerned with the amount
> > of time taken to collect our large on-chip memory.  We see improvement
> > in doing 256-bit reads at a time.
> 
> Two other options:
> 
> 1) Get the device to DMA into host memory.
> 

Unfortunately, our device doesn't support doing DMA of on-chip memory.

> 2) Use mmap() (and vm_iomap_memory() in your driver) to get direct
>    userspace access to the (I assume) PCIe memory space.
>    You can then use whatever copy instructions the cpu has.
>    (Just don't use memcpy().)
> 

We also need to collect this in kernel space i.e. from crash recovery
kernel.

Thanks,
Rahul