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:

quoted

Date: Mon, 19 Nov 2018 11:49:54 -0700
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Kenneth Lee <redacted>
CC: Leon Romanovsky <leon@kernel.org>, Kenneth Lee <redacted>,
 Tim Sell [off-list ref], linux-doc@vger.kernel.org, Alexander
 Shishkin [off-list ref], Zaibo Xu
 [off-list ref], zhangfei.gao@foxmail.com, linuxarm@huawei.com,
 haojian.zhuang@linaro.org, Christoph Lameter [off-list ref], Hao Fang
 [off-list ref], Gavin Schenk [off-list ref], RDMA mailing
 list [off-list ref], Zhou Wang [off-list ref],
 Doug Ledford [off-list ref], Uwe Kleine-König
 [off-list ref], David Kershner
 [off-list ref], Johan Hovold [off-list ref], Cyrille
 Pitchen [off-list ref], Sagar Dharia
 [off-list ref], Jens Axboe [off-list ref],
 guodong.xu@linaro.org, linux-netdev [off-list ref], Randy Dunlap
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Subject: Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce
User-Agent: Mutt/1.9.4 (2018-02-28)
Message-ID: [ref]

On Mon, Nov 19, 2018 at 05:14:05PM +0800, Kenneth Lee wrote:
 

quoted

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.

quoted

quoted

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?

quoted

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.

quoted

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...

quoted

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