Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Mon, Oct 11, 2021 at 05:02:01PM +1100, David Gibson wrote:
> qemu wants to emulate a PAPR vIOMMU, so it says (via interfaces yet to
> be determined) that it needs an IOAS where things can be mapped in the
> range 0..2GiB (for the 32-bit window) and 2^59..2^59+1TiB (for the
> 64-bit window).
> 
> Ideally the host /dev/iommu will say "ok!", since both those ranges
> are within the 0..2^60 translated range of the host IOMMU, and don't
> touch the IO hole.  When the guest calls the IO mapping hypercalls,
> qemu translates those into DMA_MAP operations, and since they're all
> within the previously verified windows, they should work fine.

Seems like we don't need the negotiation part?  The host kernel
communicates available IOVA ranges to userspace including holes (patch
17), and userspace can check that the ranges it needs are within the IOVA
space boundaries. That part is necessary for DPDK as well since it needs
to know about holes in the IOVA space where DMA wouldn't work as expected
(MSI doorbells for example). And there already is a negotiation happening,
when the host kernel rejects MAP ioctl outside the advertised area.

Thanks,
Jean