[RFC PATCH v2] uacce: Add uacce_ctrl misc device

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Tian, Kevin [mailto:kevin.tian at intel.com]
> Sent: Friday, January 29, 2021 11:09 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua at hisilicon.com>; Jason Gunthorpe
> <jgg at ziepe.ca>
> Cc: chensihang (A) <chensihang1 at hisilicon.com>; Arnd Bergmann
> <arnd at arndb.de>; Greg Kroah-Hartman <gregkh at linuxfoundation.org>;
> linux-kernel at vger.kernel.org; iommu at lists.linux-foundation.org;
> linux-mm at kvack.org; Zhangfei Gao <zhangfei.gao at linaro.org>; Liguozhu
> (Kenneth) <liguozhu at hisilicon.com>; linux-accelerators at lists.ozlabs.org
> Subject: RE: [RFC PATCH v2] uacce: Add uacce_ctrl misc device
> 
> > From: Song Bao Hua (Barry Song)
> > Sent: Tuesday, January 26, 2021 9:27 AM
> >
> > > -----Original Message-----
> > > From: Jason Gunthorpe [mailto:jgg at ziepe.ca]
> > > Sent: Tuesday, January 26, 2021 2:13 PM
> > > To: Song Bao Hua (Barry Song) <song.bao.hua at hisilicon.com>
> > > Cc: Wangzhou (B) <wangzhou1 at hisilicon.com>; Greg Kroah-Hartman
> > > <gregkh at linuxfoundation.org>; Arnd Bergmann <arnd at arndb.de>;
> > Zhangfei Gao
> > > <zhangfei.gao at linaro.org>; linux-accelerators at lists.ozlabs.org;
> > > linux-kernel at vger.kernel.org; iommu at lists.linux-foundation.org;
> > > linux-mm at kvack.org; Liguozhu (Kenneth) <liguozhu at hisilicon.com>;
> > chensihang
> > > (A) <chensihang1 at hisilicon.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.
> >
> > 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.
> >
> > And then we move to SVA case, we can still have a memory pool
> > by hugetlb or pin, and app can allocate memory from this pool
> > since this pool is mapped to the address space of the process,
> > and we are able to get stable I/O performance since it is always
> > there. But in this case, device is using the page table of
> > process with the full permission control.
> > And they are using same address and can possibly enjoy the easy
> > programming if HW supports.
> >
> > SVA is not doom to work with IO page fault only. If we have SVA+pin,
> > we would get both sharing address and stable I/O latency.
> >
> 
> Isn't it like a traditional MAP_DMA API (imply pinning) plus specifying
> cpu_va of the memory pool as the iova?

I think it enjoys the advantage of stable I/O latency of
traditional MAP_DMA, and also uses the process page table
which SVA can provide. The major difference is that in
SVA case, iova totally belongs to process and is as normal
as other heap/stack/data:
p = mmap(.....MAP_ANON....);
ioctl(/dev/acc, p, PIN);

SVA for itself, provides the ability to guarantee the
address space isolation of multiple processes.  If the
device can access the data struct  such as list, tree
directly, they can further enjoy the convenience of
programming SVA gives.

So we are looking for a combination of stable io latency
of traditional DMA map and the ability of SVA.

> 
> Thanks
> Kevin

Thanks
Barry