linux-kernel - RE: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID

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Message-ID: <BN9PR11MB54333A0E5C7BD4F8996693278CB79@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Wed, 13 Oct 2021 07:07:34 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>
CC:     Jason Gunthorpe <jgg@...dia.com>, "Liu, Yi L" <yi.l.liu@...el.com>,
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Subject: RE: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE

> From: Jean-Philippe Brucker <jean-philippe@...aro.org>
> Sent: Monday, October 11, 2021 4:50 PM
> 
> 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.
> 

Agree. This can cover the ppc platforms with fixed reserved ranges.
It's meaningless to have user further tell kernel that it is only willing
to use a subset of advertised area. for ppc platforms with dynamic
reserved ranges which are claimed by user, we can leave it out of
the common set and handled in a different way, either leveraging
ioas nesting if applied or having ppc specific cmd.

Thanks
Kevin