linux-kernel - RE: [PATCH v3 07/17] iommufd: Add IOMMU_RESV_IOVA

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Message-ID: <BN9PR11MB52764C401206FE8D7A4731628C0BA@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 2 Aug 2023 01:09:28 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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        "Duan, Zhenzhong" <zhenzhong.duan@...el.com>
Subject: RE: [PATCH v3 07/17] iommufd: Add IOMMU_RESV_IOVA_RANGES

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, August 2, 2023 2:23 AM
> 
> On Tue, Aug 01, 2023 at 02:40:44AM +0000, Tian, Kevin wrote:
> 
> > > So, I guess we should leave it like this?
> > >
> >
> > Yes. Along with this discussion (including what you explained for sw_msi)
> > let's abandon this new cmd and leave it as today.
> 
> You sure? This makes it basically impossible to write a "correct" vmm
> that is aware of what the physical memory map must be early on
> 

emmm... I thought it's what you meant by "leave it like this" and the
fact that existing VMM's memory layout happens to match the reserved
regions. Nobody complains lacking of such a interface for years then
we may postpone supporting it until it's really required.

btw even if we add this new cmd now, getting the Qemu support to
use the aggregated list when creating the guest memory map is not
a simple task given currently vfio only passively acts on change
notifications in the guest memory layout. It requires a new mechanism
to enforce strict order (probe all vfio devices before creating the memory
layout) and then injects vfio reserved regions into the layout.

Preferably let's not making it a hard dependency for this series.