Re: Light summary of LPC discussions on io page table

From: Boris Brezillon <boris.brezillon@collabora.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: "Robin Murphy" <robin.murphy@arm.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	iommu@lists.linux.dev, "Will Deacon" <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	"Rob Clark" <robdclark@gmail.com>,
	"Gaurav Kohli" <quic_gkohli@quicinc.com>,
	"Steven Price" <steven.price@arm.com>,
	"Pasha Tatashin" <pasha.tatashin@soleen.com>,
	"Sean Christopherson" <seanjc@google.com>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	"Krzysztof Wilczyński" <kwilczynski@kernel.org>
Subject: Re: Light summary of LPC discussions on io page table
Date: Thu, 23 Nov 2023 16:48:43 +0100	[thread overview]
Message-ID: <20231123164843.040a2c67@collabora.com> (raw)
In-Reply-To: <20231123151118.GA436110@nvidia.com>

Hi Jason,

On Thu, 23 Nov 2023 11:11:18 -0400
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Thu, Nov 23, 2023 at 09:51:41AM +0100, Boris Brezillon wrote:
> 
> > Can these people please raise their voice here? Or maybe LPC's IOMMU
> > discussions were summarized somewhere in an email sent to the IOMMU ML,
> > where we could discuss that, instead of this thread.  
> 
> I think we are using session recordings mostly these days? I have a
> bunch of people interested - I'm going to work to launch another group
> project to tackle this. Maybe in Jan.
> 
> The recordings are not broken up yet, but here is the master:
> 
> https://www.youtube.com/live/vO2mAM8wOvU
> 
> Pasha's talk is first at 2:21, and there was some discussion in the
> room that may be hard to hear including the use of RCU and the
> refcount. Pasha is interested in solving the memory waste his VFIO
> application causes which is basically an algorithmic problem inside
> the io page table.  There was also some side discussion about
> optimizations. mm implements a batch free technique that could apply
> here too, and a batch alloc is also potentially interesting.
> 
> My talk starts at 3:11 and we have another discussion later around
> this issue in the context of dirty tracking optimization. Joao has
> written and shared some code for what is concerning there in an
> earlier email Sean C is the voice at the back sharing the KVM
> learnings that getting to a common implementation is highly
> recommended
> 
> I had an interesting hallway discussion about a use case for bringing
> back the old VFIO type 1.0 huge page split behavior and implementing
> it in every enterprise iommu.
> 
> Then we have this DRM need, which I've come to think of as a
> guarenteed map without a table level allocate.
> 
> These need touching all the "enterprise" focused page table
> formats. There are so many now, doing the algorithm work 5-7 times is
> not reasonable. A common radix algorithm and a more complex shared
> implementation relying on RCU is the direction I want to head in. mm
> shows a template how to achieve this, and I have some rough ideas to
> explore.

Thanks for pulling this together. I'll try to watch the video and make
sense of what's explained there. I'll get back to you if I have
questions.

Regards,

Boris