RE: [RFC PATCH v2] uacce: Add uacce_ctrl misc device

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Tian, Kevin [mailto:kevin.tian@intel.com]
> Sent: Tuesday, February 2, 2021 3:52 PM
> To: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>; chensihang (A)
> <chensihang1@hisilicon.com>; Arnd Bergmann <arnd@arndb.de>; Greg
> Kroah-Hartman <gregkh@linuxfoundation.org>; linux-kernel@vger.kernel.org;
> iommu@lists.linux-foundation.org; linux-mm@kvack.org; Zhangfei Gao
> <zhangfei.gao@linaro.org>; Liguozhu (Kenneth) <liguozhu@hisilicon.com>;
> linux-accelerators@lists.ozlabs.org
> Subject: RE: [RFC PATCH v2] uacce: Add uacce_ctrl misc device
> 
> > From: Jason Gunthorpe <jgg@ziepe.ca>
> > Sent: Tuesday, February 2, 2021 7:44 AM
> >
> > On Fri, Jan 29, 2021 at 10:09:03AM +0000, Tian, Kevin wrote:
> > > > 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 their issue is the HW can't do the cpu_va trick without also
> > involving the system IOMMU in a SVA mode
> >
> 
> This is the part that I didn't understand. Using cpu_va in a MAP_DMA
> interface doesn't require device support. It's just an user-specified
> address to be mapped into the IOMMU page table. On the other hand,

The background is that uacce is based on SVA and we are building
applications on uacce:
https://www.kernel.org/doc/html/v5.10/misc-devices/uacce.html
so IOMMU simply uses the page table of MMU, and don't do any
special mapping to an user-specified address. We don't break
the basic assumption that uacce is using SVA, otherwise, we
need to re-build uacce and the whole base.

> sharing CPU page table through a SVA interface for an usage where I/O
> page faults must be completely avoided seems a misleading attempt.

That is not for completely avoiding IO page fault, that is just
an extension for high-performance I/O case, providing a way to
avoid IO latency jitter. Using it or not is totally up to users.

> Even if people do want this model (e.g. mix pinning+fault), it should be
> a mm syscall as Greg pointed out, not specific to sva.
> 

We are glad to make it a syscall if people are happy with
it. The simplest way would be a syscall similar with
userfaultfd  if we don't want to mess up mm_struct.

> Thanks
> Kevin

Thanks
Barry