linux-kernel - Re: [PATCH v2 1/2] iommu/virtio: Make use of ops->iotlb_sync

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Date:   Fri, 22 Sep 2023 08:57:19 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Robin Murphy <robin.murphy@....com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        virtualization@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iommu/virtio: Make use of ops->iotlb_sync_map

On Tue, Sep 19, 2023 at 11:46:49AM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 19, 2023 at 09:15:19AM +0100, Jean-Philippe Brucker wrote:
> > On Mon, Sep 18, 2023 at 05:37:47PM +0100, Robin Murphy wrote:
> > > > diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> > > > index 17dcd826f5c2..3649586f0e5c 100644
> > > > --- a/drivers/iommu/virtio-iommu.c
> > > > +++ b/drivers/iommu/virtio-iommu.c
> > > > @@ -189,6 +189,12 @@ static int viommu_sync_req(struct viommu_dev *viommu)
> > > >   	int ret;
> > > >   	unsigned long flags;
> > > > +	/*
> > > > +	 * .iotlb_sync_map and .flush_iotlb_all may be called before the viommu
> > > > +	 * is initialized e.g. via iommu_create_device_direct_mappings()
> > > > +	 */
> > > > +	if (!viommu)
> > > > +		return 0;
> > > 
> > > Minor nit: I'd be inclined to make that check explicitly in the places where
> > > it definitely is expected, rather than allowing *any* sync to silently do
> > > nothing if called incorrectly. Plus then they could use
> > > vdomain->nr_endpoints for consistency with the equivalent checks elsewhere
> > > (it did take me a moment to figure out how we could get to .iotlb_sync_map
> > > with a NULL viommu without viommu_map_pages() blowing up first...)
> 
> This makes more sense to me
> 
> Ultimately this driver should reach a point where every iommu_domain
> always has a non-null domain->viommu because it will be set during
> alloc.
> 
> But it can still have nr_endpoints == 0, doesn't it make sense to
> avoid sync in this case?
> 
> (btw this driver is missing locking around vdomain->nr_endpoints)

Yes, that's on my list

> 
> > They're not strictly equivalent: this check works around a temporary issue
> > with the IOMMU core, which calls map/unmap before the domain is
> > finalized.
> 
> Where? The above points to iommu_create_device_direct_mappings() but
> it doesn't because the pgsize_bitmap == 0:

__iommu_domain_alloc() sets pgsize_bitmap in this case:

        /*
         * If not already set, assume all sizes by default; the driver
         * may override this later
         */
        if (!domain->pgsize_bitmap)
                domain->pgsize_bitmap = bus->iommu_ops->pgsize_bitmap;

Thanks,
Jean