linux-kernel - Re: [PATCH v2 1/3] iommufd: Add devices_users to track the hw

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Message-ID: <Y9gi0UaE1PlKVzmn@Asurada-Nvidia>
Date:   Mon, 30 Jan 2023 12:04:33 -0800
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     <kevin.tian@...el.com>, <yi.l.liu@...el.com>,
        <iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] iommufd: Add devices_users to track the
 hw_pagetable usage by device

On Mon, Jan 30, 2023 at 03:50:07PM -0400, Jason Gunthorpe wrote:
> On Mon, Jan 30, 2023 at 11:27:37AM -0800, Nicolin Chen wrote:
> > On Mon, Jan 30, 2023 at 11:02:25AM -0400, Jason Gunthorpe wrote:
> > > On Sat, Jan 28, 2023 at 01:18:09PM -0800, Nicolin Chen wrote:
> > > > From: Yi Liu <yi.l.liu@...el.com>
> > > > 
> > > > Currently, hw_pagetable tracks the attached devices using a device list.
> > > > When attaching the first device to the kernel-managed hw_pagetable, it
> > > > should be linked to IOAS. When detaching the last device from this hwpt,
> > > > the link with IOAS should be removed too. And this first-or-last device
> > > > check is done with list_empty(hwpt->devices).
> > > > 
> > > > However, with a nested configuration, when a device is attached to the
> > > > user-managed stage-1 hw_pagetable, it will be added to this user-managed
> > > > hwpt's device list instead of the kernel-managed stage-2 hwpt's one. And
> > > > this breaks the logic for a kernel-managed hw_pagetable link/disconnect
> > > > to/from IOAS/IOPT. e.g. the stage-2 hw_pagetable would be linked to IOAS
> > > > multiple times if multiple device is attached, but it will become empty
> > > > as soon as one device detached.
> > > 
> > > Why this seems really weird to say.
> > > 
> > > The stage 2 is linked explicitly to the IOAS that drives it's
> > > map/unmap
> > > 
> > > Why is there any implicit activity here? There should be no implicit
> > > attach of the S2 to an IOAS ever.
> > 
> > I think this is supposed to say the following use case:
> > 
> > Two stage-1 hwpts share the same parent s2_hwpt:
> > 
> > attach device1 to stage-1 hwpt1:
> > 	...
> > 	if (list_empty(s1_hwpt1->devices))		// empty; true
> > 		iopt_table_add_domain(s2_hwpt->domain); // do once
> > 	s1_hwpt1 device list cnt++;
> > 	...
> 
> No, this doesn't make sense.
> 
> The s2_hwpt should be created explicitly, not using autodomains

iopt_table_add_domain() is called in iommufd_device_do_attach(),
so it's shared by both a created hwpt or autodomain.

> When it is created it should be linked to a single IOAS and that is
> when iopt_table_add_domain() should have been called.

I recall we've discussed this that SMMU sets up domain when it
attaches the device to, so we made a compromise here...

> The S1 attach should do *nothing* to a S2.

With that compromise, a nested attach flow may be
1) create an s2 hwpt
2) create an s1 hwpt
3) attach dev to s1
    calls iopt_table_add_domain()

Thanks
Nicolin