linux-kernel - Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

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Date:   Tue, 8 May 2018 18:17:33 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christian König <christian.koenig@....com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all
 devices behind switches

On Tue, 8 May 2018 17:31:48 -0600
Logan Gunthorpe <logang@...tatee.com> wrote:
> On 08/05/18 05:11 PM, Alex Williamson wrote:
> > A runtime, sysfs approach has some benefits here,
> > especially in identifying the device assuming we're ok with leaving
> > the persistence problem to userspace tools.  I'm still a little fond of
> > the idea of exposing an acs_flags attribute for devices in sysfs where
> > a write would do a soft unplug and re-add of all affected devices to
> > automatically recreate the proper grouping.  Any dynamic change in
> > routing and grouping would require all DMA be re-established anyway and
> > a soft hotplug seems like an elegant way of handling it.  Thanks,  
> 
> This approach sounds like it has a lot more issues to contend with:
> 
> For starters, a soft unplug/re-add of all the devices behind a switch is
> going to be difficult if a lot of those devices have had drivers
> installed and their respective resources are now mounted or otherwise in
> use.
> 
> Then, do we have to redo a the soft-replace every time we change the ACS
> bit for every downstream port? That could mean you have to do dozens
> soft-replaces before you have all the ACS bits set which means you have
> a storm of drivers being added and removed.

True, anything requiring tweaking multiple downstream ports would
induce a hot-unplug/replug for each.  A better sysfs interface would
allow multiple downstream ports to be updated in a single shot.

> This would require some kind of fancy custom setup software that runs at
> just the right time in the boot sequence or a lot of work on the users
> part to unbind all the resources, setup the ACS bits and then rebind
> everything (assuming the soft re-add doesn't rebind it every time you
> adjust one ACS bit). Ugly.
> 
> IMO, if we need to do the sysfs approach then we need to be able to
> adjust the groups dynamically in a sensible way and not through the
> large hammer that is soft-replaces. I think this would be great but I
> don't think we will be tackling that with this patch set.

OTOH, I think the only sensible way to dynamically adjust groups is
through hotplug, we cannot have running drivers attached to downstream
endpoints as we're adjusting the routing.  Thanks,

Alex