linux-kernel - Re: [PATCH v3 04/11] iommufd/viommu: Add IOMMU_VIOMMU

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Message-ID: <20241021121944.GO3559746@nvidia.com>
Date: Mon, 21 Oct 2024 09:19:44 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Alexey Kardashevskiy <aik@....com>
Cc: Nicolin Chen <nicolinc@...dia.com>, kevin.tian@...el.com,
	will@...nel.org, joro@...tes.org, suravee.suthikulpanit@....com,
	robin.murphy@....com, dwmw2@...radead.org, baolu.lu@...ux.intel.com,
	shuah@...nel.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, eric.auger@...hat.com,
	jean-philippe@...aro.org, mdf@...nel.org, mshavit@...gle.com,
	shameerali.kolothum.thodi@...wei.com, smostafa@...gle.com,
	yi.l.liu@...el.com, patches@...ts.linux.dev
Subject: Re: [PATCH v3 04/11] iommufd/viommu: Add IOMMU_VIOMMU_ALLOC ioctl

On Mon, Oct 21, 2024 at 07:11:47PM +1100, Alexey Kardashevskiy wrote:
> > +	hwpt_paging = iommufd_get_hwpt_paging(ucmd, cmd->hwpt_id);
> > +	if (IS_ERR(hwpt_paging)) {
> > +		rc = PTR_ERR(hwpt_paging);
> 
> 
> iommufd_get_hwpt_paging() is container_of() which does not test for the
> value and just does a simple math so the actual error value from
> iommufd_get_object() is ... lost?
> 
> Oh well, not really lost, as "obj" and "common.obj" seem to be forced to be
> at the beginning of iommufd object structs so container_of() is no-op or is
> not it?

Yes, it is just a type check, the revised code I suggested doesn't
rely on this. Jonathon remarked on the same for fwctl and we came up
with something different.

> If something needs to be guaranteed to stay alive, may be just call
> device_get(viommu->dev), with the comment above? Or it is some different
> refcounting which is missing? Thanks,

device_get is different, it just makes sure the struct device memory
is around, it doesn't prevent iommu unplug

Jason