linux-kernel - Re: [PATCH 06/17] iommufd/hw_pagetable: Use domain_alloc

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Message-ID: <Y+VOw6dTnGapMm9L@Asurada-Nvidia>
Date:   Thu, 9 Feb 2023 11:51:31 -0800
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>, Yi Liu <yi.l.liu@...el.com>
CC:     Matthew Rosato <mjrosato@...ux.ibm.com>, <joro@...tes.org>,
        <alex.williamson@...hat.com>, <kevin.tian@...el.com>,
        <robin.murphy@....com>, <cohuck@...hat.com>,
        <eric.auger@...hat.com>, <kvm@...r.kernel.org>,
        <chao.p.peng@...ux.intel.com>, <yi.y.sun@...ux.intel.com>,
        <peterx@...hat.com>, <jasowang@...hat.com>,
        <shameerali.kolothum.thodi@...wei.com>, <lulu@...hat.com>,
        <suravee.suthikulpanit@....com>, <iommu@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH 06/17] iommufd/hw_pagetable: Use domain_alloc_user op for
 domain allocation

On Thu, Feb 09, 2023 at 02:36:49PM -0400, Jason Gunthorpe wrote:
> On Thu, Feb 09, 2023 at 12:59:58PM -0500, Matthew Rosato wrote:
> > really should highlight that).  Otherwise, conditionally calling
> > iommu_domain_alloc(dev->bus) when !ops->domain_alloc_user (instead
> > of returning -EOPNOTSUPP) seems to restore the prior functionality
> > for me.
> 
> Yes, that is right if the input user data is 0 length or full of 0s
> then we should call the normal driver function

Maybe I am wrong, yet I recall that doing ops->domain_alloc_user
without a fallback was intentional to rule out drivers that don't
support IOMMUFD?

To be backward-compatible and concern about SMMU, we can opt in
ops->domain_alloc_user upon availability and then add a fallback:

	if ((!ops || !ops->domain_alloc_user) && user_data) {
		rc = -EOPNOTSUPP;
		goto out_abort;
	}

	if (ops->domain_alloc_user)
		hwpt->domain = ops->domain_alloc_user(dev, NULL, NULL);
	else
		hwpt->domain = iommu_domain_alloc(dev->bus);
	if (!hwpt->domain) {
		rc = -ENOMEM;
		goto out_abort;
	}

Yet, even by doing so, this series having the PATCH 07/17 that
moves iopt_table_add_domain() would temporally break IOMMUFD on
ARM platforms, until we add the ops->domain_alloc_user to SMMU
drivers.

Thanks
Nic