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

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Message-ID: <20230922233309.GI13795@ziepe.ca>
Date:   Fri, 22 Sep 2023 20:33:09 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        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 Fri, Sep 22, 2023 at 07:07:40PM +0100, Robin Murphy wrote:

> virtio isn't setting ops->pgsize_bitmap for the sake of direct mappings
> either; it sets it once it's discovered any instance, since apparently it's
> assuming that all instances must support identical page sizes, and thus once
> it's seen one it can work "normally" per the core code's assumptions. It's
> also I think the only driver which has a "finalise" bodge but *can* still
> properly support map-before-attach, by virtue of having to replay mappings
> to every new endpoint anyway.

Well it can't quite do that since it doesn't know the geometry - it
all is sort of guessing and hoping it doesn't explode on replay. If it
knows the geometry it wouldn't need finalize...

> > What do you think about something like this to replace
> > iommu_create_device_direct_mappings(), that does enforce things
> > properly?
> 
> I fail to see how that would make any practical difference. Either the
> mappings can be correctly set up in a pagetable *before* the relevant device
> is attached to that pagetable, or they can't (if the driver doesn't have
> enough information to be able to do so) and we just have to really hope
> nothing blows up in the race window between attaching the device to an empty
> pagetable and having a second try at iommu_create_device_direct_mappings().
> That's a driver-level issue and has nothing to do with pgsize_bitmap either
> way.

Except we don't detect this in the core code correctly, that is my
point. We should detect the aperture conflict, not pgsize_bitmap to
check if it is the first or second try.

Jason