linux-kernel - Re: [RFC PATCH v2] uacce: Add uacce

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Date:   Mon, 25 Jan 2021 21:13:04 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     "Wangzhou (B)" <wangzhou1@...ilicon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        "linux-accelerators@...ts.ozlabs.org" 
        <linux-accelerators@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Liguozhu (Kenneth)" <liguozhu@...ilicon.com>,
        "chensihang (A)" <chensihang1@...ilicon.com>
Subject: Re: [RFC PATCH v2] uacce: Add uacce_ctrl misc device

On Mon, Jan 25, 2021 at 11:35:22PM +0000, Song Bao Hua (Barry Song) wrote:

> > On Mon, Jan 25, 2021 at 10:21:14PM +0000, Song Bao Hua (Barry Song) wrote:
> > > mlock, while certainly be able to prevent swapping out, it won't
> > > be able to stop page moving due to:
> > > * memory compaction in alloc_pages()
> > > * making huge pages
> > > * numa balance
> > > * memory compaction in CMA
> > 
> > Enabling those things is a major reason to have SVA device in the
> > first place, providing a SW API to turn it all off seems like the
> > wrong direction.
> 
> I wouldn't say this is a major reason to have SVA. If we read the
> history of SVA and papers, people would think easy programming due
> to data struct sharing between cpu and device, and process space
> isolation in device would be the major reasons for SVA. SVA also
> declares it supports zero-copy while zero-copy doesn't necessarily
> depend on SVA.

Once you have to explicitly make system calls to declare memory under
IO, you loose all of that.

Since you've asked the app to be explicit about the DMAs it intends to
do, there is not really much reason to use SVA for those DMAs anymore.

Jason