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

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Date:   Tue, 26 Jan 2021 14:20:24 -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 Tue, Jan 26, 2021 at 01:26:45AM +0000, Song Bao Hua (Barry Song) wrote:
> > 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.
> 
> Let's see a non-SVA case. We are not using SVA, we can have
> a memory pool by hugetlb or pin, and app can allocate memory
> from this pool, and get stable I/O performance on the memory
> from the pool. But device has its separate page table which
> is not bound with this process, thus lacking the protection
> of process space isolation. Plus, CPU and device are using
> different address.

So you are relying on the platform to do the SVA for the device?

This feels like it goes back to another topic where I felt the SVA
setup uAPI should be shared and not buried into every driver's unique
ioctls.

Having something like this in a shared SVA system is somewhat less
strange.

Jason