linux-kernel - Re: [PATCH v3 08/17] iommufd: IOMMU_HWPT

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Message-ID: <ZMe0IQksFgV6yKTB@nvidia.com>
Date:   Mon, 31 Jul 2023 10:16:17 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "yi.y.sun@...ux.intel.com" <yi.y.sun@...ux.intel.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Duan, Zhenzhong" <zhenzhong.duan@...el.com>
Subject: Re: [PATCH v3 08/17] iommufd: IOMMU_HWPT_ALLOC allocation with user
 data

On Mon, Jul 31, 2023 at 06:31:20AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Saturday, July 29, 2023 1:56 AM
> > 
> > On Mon, Jul 24, 2023 at 04:03:57AM -0700, Yi Liu wrote:
> > 
> > > +	switch (pt_obj->type) {
> > > +	case IOMMUFD_OBJ_IOAS:
> > > +		ioas = container_of(pt_obj, struct iommufd_ioas, obj);
> > > +		break;
> > > +	case IOMMUFD_OBJ_HW_PAGETABLE:
> > > +		/* pt_id points HWPT only when hwpt_type
> > is !IOMMU_HWPT_TYPE_DEFAULT */
> > > +		if (cmd->hwpt_type == IOMMU_HWPT_TYPE_DEFAULT) {
> > > +			rc = -EINVAL;
> > > +			goto out_put_pt;
> > > +		}
> > > +
> > > +		parent = container_of(pt_obj, struct iommufd_hw_pagetable,
> > obj);
> > > +		/*
> > > +		 * Cannot allocate user-managed hwpt linking to
> > auto_created
> > > +		 * hwpt. If the parent hwpt is already a user-managed hwpt,
> > > +		 * don't allocate another user-managed hwpt linking to it.
> > > +		 */
> > > +		if (parent->auto_domain || parent->parent) {
> > > +			rc = -EINVAL;
> > > +			goto out_put_pt;
> > > +		}
> > > +		ioas = parent->ioas;
> > 
> > Why do we set ioas here? I would think it should be NULL.
> > 
> > I think it is looking like a mistake to try and re-use
> > iommufd_hw_pagetable_alloc() directly for the nested case. It should
> > not have a IOAS argument, it should not do enforce_cc, or iopt_*
> > functions
> 
> enforce_cc is still required since it's about memory accesses post
> page table walking (no matter the walked table is single stage or
> nested).

enforce_cc only has meaning if the kernel owns the IOPTEs, and if we
are creating a nest it does not. The guest has to set any necessary
IOPTE bits.

enforce_cc will be done on the parent of the nest as normal.

> Ideally expanding uAPI structure size should come with new flag bits.

Flags or some kind of 'zero is the same behavior as a smaller struct'
scheme.

This patch is doing the zero option:

 	__u32 __reserved;
+	__u32 hwpt_type;
+	__u32 data_len;
+	__aligned_u64 data_uptr;
 };

hwpt_type == 0 means default type
data_len == 0 means no data
data_uptr is ignored (zero is safe)

So there is no need to change it

Jason