linux-kernel - RE: [PATCH v2 1/2] iommu: Prevent RESV_DIRECT devices from blocking domains

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Date:   Mon, 24 Jul 2023 02:15:40 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "Will Deacon" <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        "Alex Williamson" <alex.williamson@...hat.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 1/2] iommu: Prevent RESV_DIRECT devices from blocking
 domains

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Friday, July 21, 2023 11:10 PM
> 
> On Fri, Jul 21, 2023 at 03:07:47AM +0000, Tian, Kevin wrote:
> > > @@ -974,13 +972,17 @@ static int
> > > iommu_create_device_direct_mappings(struct iommu_domain *domain,
> > >  		dma_addr_t start, end, addr;
> > >  		size_t map_size = 0;
> > >
> > > +		if (entry->type == IOMMU_RESV_DIRECT)
> > > +			dev->iommu->requires_direct = 1;
> > > +
> > > +		if ((entry->type != IOMMU_RESV_DIRECT &&
> > > +		     entry->type != IOMMU_RESV_DIRECT_RELAXABLE) ||
> > > +		    !iommu_is_dma_domain(domain))
> > > +			continue;
> >
> > piggybacking a device attribute detection in a function which tries to
> > populate domain mappings is a bit confusing.
> 
> It is, but to do otherwise we'd want to have the caller obtain the
> reserved regions list and iterate it twice. Not sure it is worth the
> trouble right now.
> 

Not a strong opinion but It's a slow path and readability is more
preferable to me. 😊