On 2023-09-23 00:33, Jason Gunthorpe wrote:
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...
I think it's entirely reasonable to assume that any direct mappings
specified for a device are valid for that device and its IOMMU. However,
in the particular case of virtio, it really shouldn't ever have direct
mappings anyway, since even if the underlying hardware did have any, the
host can enforce the actual direct-mapping aspect itself, and just
present them as unusable regions to the guest.
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.
Again, that's irrelevant. It can only be about whether the actual
->map_pages call succeeds or not. A driver could well know up-front that
all instances support the same pgsize_bitmap and aperture, and set both
at ->domain_alloc time, yet still be unable to handle an actual mapping
without knowing which instance(s) that needs to interact with (e.g.
omap-iommu).
Thanks,
Robin.
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