linux-kernel - RE: [PATCH v4 02/11] iommufd: Introduce IOMMUFD_OBJ

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Message-ID: <BN9PR11MB527637A495A46F32F722FB9C8C4F2@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 25 Oct 2024 08:47:40 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>, Baolu Lu <baolu.lu@...ux.intel.com>
CC: Nicolin Chen <nicolinc@...dia.com>, "will@...nel.org" <will@...nel.org>,
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Subject: RE: [PATCH v4 02/11] iommufd: Introduce IOMMUFD_OBJ_VIOMMU and its
 related struct

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, October 22, 2024 9:16 PM
> 
> On Tue, Oct 22, 2024 at 04:59:07PM +0800, Baolu Lu wrote:
> 
> > Is it feasible to make vIOMMU object more generic, rather than strictly
> > tying it to nested translation? For example, a normal paging domain that
> > translates gPAs to hPAs could also have a vIOMMU object associated with
> > it.
> >
> > While we can only support vIOMMU object allocation uAPI for S2 paging
> > domains in the context of this series, we could consider leaving the
> > option open to associate a vIOMMU object with other normal paging
> > domains that are not a nested parent?
> 
> Why? The nested parent flavour of the domain is basically free to
> create, what reason would be to not do that?
> 
> If the HW doesn't support it, then does the HW really need/support a
> VIOMMU?
> 

Now it's agreed to build trusted I/O on top of this new vIOMMU object.
format-wise probably it's free to assume that nested parent is supported
on any new platform which will support trusted I/O. But I'm not sure
all the conditions around allowing nested are same as for trusted I/O,
e.g. for ARM nesting is allowed only for CANWBS/S2FWB. Are they
always guaranteed in trusted I/O configuration?

Baolu did raise a good open to confirm given it will be used beyond
nesting. 😊