linux-kernel - Re: [PATCH v12 07/17] iommu: Try to allocate blocking domain when probing device

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Message-ID: <Ywz3A3tHEzY+Sf7b@nvidia.com>
Date:   Mon, 29 Aug 2022 14:27:31 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhu Tony <tony.zhu@...el.com>, iommu@...ts.linux.dev,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 07/17] iommu: Try to allocate blocking domain when
 probing device

On Mon, Aug 29, 2022 at 11:40:24AM +0800, Baolu Lu wrote:
> On 2022/8/26 22:52, Jason Gunthorpe wrote:
> > On Fri, Aug 26, 2022 at 08:11:31PM +0800, Lu Baolu wrote:
> > > Allocate the blocking domain when probing devices if the driver supports
> > > blocking domain allocation. Otherwise, revert to the previous behavior,
> > > that is, use UNMANAGED domain instead when the blocking domain is needed.
> > > 
> > > Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> > > Tested-by: Zhangfei Gao<zhangfei.gao@...aro.org>
> > > Tested-by: Tony Zhu<tony.zhu@...el.com>
> > > ---
> > >   drivers/iommu/iommu.c | 29 +++++++++++++++++------------
> > >   1 file changed, 17 insertions(+), 12 deletions(-)
> > This seems like a lot of overhead to allocate these things for every
> > group?
> > 
> > Why not add a simple refcount on the blocking domain instead and
> > allocate the domain on the pasid attach like we do for ownership?
> 
> I am working towards implementing static instance of blocking domain for
> each IOMMU driver, and then, there's no much overhead to allocate it in
> the probing device path.

Well, I thought about that and I don't think we can get
there in a short order. Would rather you progress this series without
getting entangled in such a big adventure

Jason