Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce

[Jonathan Cameron]

+CC Jean-Phillipe and iommu list.


On Mon, 19 Nov 2018 20:29:39 -0700
Jason Gunthorpe <jgg@xxxxxxxx> wrote:

> On Tue, Nov 20, 2018 at 11:07:02AM +0800, Kenneth Lee wrote:
> > On Mon, Nov 19, 2018 at 11:49:54AM -0700, Jason Gunthorpe wrote:  
> > > Date: Mon, 19 Nov 2018 11:49:54 -0700
> > > From: Jason Gunthorpe <jgg@xxxxxxxx>
> > > To: Kenneth Lee <liguozhu@xxxxxxxxxxxxx>
> > > CC: Leon Romanovsky <leon@xxxxxxxxxx>, Kenneth Lee <nek.in.cn@xxxxxxxxx>,
> > >  Tim Sell <timothy.sell@xxxxxxxxxx>, linux-doc@xxxxxxxxxxxxxxx, Alexander
> > >  Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>, Zaibo Xu
> > >  <xuzaibo@xxxxxxxxxx>, zhangfei.gao@xxxxxxxxxxx, linuxarm@xxxxxxxxxx,
> > >  haojian.zhuang@xxxxxxxxxx, Christoph Lameter <cl@xxxxxxxxx>, Hao Fang
> > >  <fanghao11@xxxxxxxxxx>, Gavin Schenk <g.schenk@xxxxxxxxxxxx>, RDMA mailing
> > >  list <linux-rdma@xxxxxxxxxxxxxxx>, Zhou Wang <wangzhou1@xxxxxxxxxxxxx>,
> > >  Doug Ledford <dledford@xxxxxxxxxx>, Uwe Kleine-König
> > >  <u.kleine-koenig@xxxxxxxxxxxxxx>, David Kershner
> > >  <david.kershner@xxxxxxxxxx>, Johan Hovold <johan@xxxxxxxxxx>, Cyrille
> > >  Pitchen <cyrille.pitchen@xxxxxxxxxxxxxxxxxx>, Sagar Dharia
> > >  <sdharia@xxxxxxxxxxxxxx>, Jens Axboe <axboe@xxxxxxxxx>,
> > >  guodong.xu@xxxxxxxxxx, linux-netdev <netdev@xxxxxxxxxxxxxxx>, Randy Dunlap
> > >  <rdunlap@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Vinod Koul
> > >  <vkoul@xxxxxxxxxx>, linux-crypto@xxxxxxxxxxxxxxx, Philippe Ombredanne
> > >  <pombredanne@xxxxxxxx>, Sanyog Kale <sanyog.r.kale@xxxxxxxxx>, "David S.
> > >  Miller" <davem@xxxxxxxxxxxxx>, linux-accelerators@xxxxxxxxxxxxxxxx
> > > Subject: Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce
> > > User-Agent: Mutt/1.9.4 (2018-02-28)
> > > Message-ID: <20181119184954.GB4890@xxxxxxxx>
> > > 
> > > On Mon, Nov 19, 2018 at 05:14:05PM +0800, Kenneth Lee wrote:
> > >    
> > > > If the hardware cannot share page table with the CPU, we then need to have
> > > > some way to change the device page table. This is what happen in ODP. It
> > > > invalidates the page table in device upon mmu_notifier call back. But this cannot
> > > > solve the COW problem: if the user process A share a page P with device, and A 
> > > > forks a new process B, and it continue to write to the page. By COW, the
> > > > process B will keep the page P, while A will get a new page P'. But you have
> > > > no way to let the device know it should use P' rather than P.  
> > > 
> > > Is this true? I thought mmu_notifiers covered all these cases.
> > > 
> > > The mm_notifier for A should fire if B causes the physical address of
> > > A's pages to change via COW. 
> > > 
> > > And this causes the device page tables to re-synchronize.  
> > 
> > I don't see such code. The current do_cow_fault() implemenation has nothing to
> > do with mm_notifer.  
> 
> Well, that sure sounds like it would be a bug in mmu_notifiers..
> 
> But considering Jean's SVA stuff seems based on mmu notifiers, I have
> a hard time believing that it has any different behavior from RDMA's
> ODP, and if it does have different behavior, then it is probably just
> a bug in the ODP implementation.
> 
> > > > In WarpDrive/uacce, we make this simple. If you support IOMMU and it support
> > > > SVM/SVA. Everything will be fine just like ODP implicit mode. And you don't need
> > > > to write any code for that. Because it has been done by IOMMU framework. If it  
> > > 
> > > Looks like the IOMMU code uses mmu_notifier, so it is identical to
> > > IB's ODP. The only difference is that IB tends to have the IOMMU page
> > > table in the device, not in the CPU.
> > > 
> > > The only case I know if that is different is the new-fangled CAPI
> > > stuff where the IOMMU can directly use the CPU's page table and the
> > > IOMMU page table (in device or CPU) is eliminated.  
> >
> > Yes. We are not focusing on the current implementation. As mentioned in the
> > cover letter. We are expecting Jean Philips' SVA patch:
> > git://linux-arm.org/linux-jpb.  
> 
> This SVA stuff does not look comparable to CAPI as it still requires
> maintaining seperate IOMMU page tables.
> 
> Also, those patches from Jean have a lot of references to
> mmu_notifiers (ie look at iommu_mmu_notifier).
> 
> Are you really sure it is actually any different at all?
> 
> > > Anyhow, I don't think a single instance of hardware should justify an
> > > entire new subsystem. Subsystems are hard to make and without multiple
> > > hardware examples there is no way to expect that it would cover any
> > > future use cases.  
> > 
> > Yes. That's our first expectation. We can keep it with our driver. But because
> > there is no user driver support for any accelerator in mainline kernel. Even the
> > well known QuickAssit has to be maintained out of tree. So we try to see if
> > people is interested in working together to solve the problem.  
> 
> Well, you should come with patches ack'ed by these other groups.
> 
> > > If all your driver needs is to mmap some PCI bar space, route
> > > interrupts and do DMA mapping then mediated VFIO is probably a good
> > > choice.   
> > 
> > Yes. That is what is done in our RFCv1/v2. But we accepted Jerome's opinion and
> > try not to add complexity to the mm subsystem.  
> 
> Why would a mediated VFIO driver touch the mm subsystem? Sounds like
> you don't have a VFIO driver if it needs to do stuff like that...
> 
> > > If it needs to do a bunch of other stuff, not related to PCI bar
> > > space, interrupts and DMA mapping (ie special code for compression,
> > > crypto, AI, whatever) then you should probably do what Jerome said and
> > > make a drivers/char/hisillicon_foo_bar.c that exposes just what your
> > > hardware does.  
> > 
> > Yes. If no other accelerator driver writer is interested. That is the
> > expectation:)  
> 
> I don't think it matters what other drivers do. 
> 
> If your driver does not need any other kernel code then VFIO is
> sensible. In this kind of world you will probably have a RDMA-like
> userspace driver that can bring this to a common user space API, even
> if one driver use VFIO and a different driver uses something else.
> 
> > You create some connections (queues) to NIC, RSA, and AI engine. Then you got
> > data direct from the NIC and pass the pointer to RSA engine for decryption. The
> > CPU then finish some data taking or operation and then pass through to the AI
> > engine for CNN calculation....This will need a place to maintain the same
> > address space by some means.  
> 
> How is this any different from what we have today? 
> 
> SVA is not something even remotely new, IB has been doing various
> versions of it for 20 years.
> 
> Jason