Re: [PATCH v2 03/40] iommu/sva: Manage process address spaces

[Kenneth Lee <Kenneth-Lee-2012@foxmail.com>]

On Fri, May 25, 2018 at 09:39:59AM +0100, Jonathan Cameron wrote:
> Date: Fri, 25 May 2018 09:39:59 +0100
> From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> To: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> CC: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
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> Subject: Re: [PATCH v2 03/40] iommu/sva: Manage process address spaces
> Message-ID: <20180525093959.000040a7@huawei.com>
> 
> +CC Kenneth Lee
> 
> On Fri, 25 May 2018 09:33:11 +0300
> Ilias Apalodimas <ilias.apalodimas@linaro.org> wrote:
> 
> > On Thu, May 24, 2018 at 04:04:39PM +0100, Jean-Philippe Brucker wrote:
> > > On 24/05/18 12:50, Ilias Apalodimas wrote:  
> > > >> Interesting, I hadn't thought about this use-case before. At first I
> > > >> thought you were talking about mdev devices assigned to VMs, but I think
> > > >> you're referring to mdevs assigned to userspace drivers instead? Out of
> > > >> curiosity, is it only theoretical or does someone actually need this?  
> > > > 
> > > > There has been some non upstreamed efforts to have mdev and produce userspace
> > > > drivers. Huawei is using it on what they call "wrapdrive" for crypto devices and
> > > > we did a proof of concept for ethernet interfaces. At the time we choose not to
> > > > involve the IOMMU for the reason you mentioned, but having it there would be
> > > > good.  
> > > 
> > > I'm guessing there were good reasons to do it that way but I wonder, is
> > > it not simpler to just have the kernel driver create a /dev/foo, with a
> > > standard ioctl/mmap/poll interface? Here VFIO adds a layer of
> > > indirection, and since the mediating driver has to implement these
> > > operations already, what is gained?  
> > The best reason i can come up with is "common code". You already have one API
> > doing that for you so we replicate it in a /dev file?
> > The mdev approach still needs extentions to support what we tried to do (i.e
> > mdev bus might need yo have access on iommu_ops), but as far as i undestand it's
> > a possible case.

Hi, Jean, Please allow me to share my understanding here:
https://zhuanlan.zhihu.com/p/35489035

The reason we do not use the /dev/foo scheme is that the devices to be
shared are programmable accelerators. We cannot fix up the kernel driver for them.
> > > 
> > > Thanks,
> > > Jean  
> 
> 

-- 
			-Kenneth Lee (Hisilicon)