Re: [RFC] /dev/ioasid uAPI proposal

[David Gibson]

On Thu, Jun 03, 2021 at 02:49:56AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Thursday, June 3, 2021 12:59 AM
> > 
> > On Wed, Jun 02, 2021 at 04:48:35PM +1000, David Gibson wrote:
> > > > > 	/* Bind guest I/O page table  */
> > > > > 	bind_data = {
> > > > > 		.ioasid	= gva_ioasid;
> > > > > 		.addr	= gva_pgtable1;
> > > > > 		// and format information
> > > > > 	};
> > > > > 	ioctl(ioasid_fd, IOASID_BIND_PGTABLE, &bind_data);
> > > >
> > > > Again I do wonder if this should just be part of alloc_ioasid. Is
> > > > there any reason to split these things? The only advantage to the
> > > > split is the device is known, but the device shouldn't impact
> > > > anything..
> > >
> > > I'm pretty sure the device(s) could matter, although they probably
> > > won't usually.
> > 
> > It is a bit subtle, but the /dev/iommu fd itself is connected to the
> > devices first. This prevents wildly incompatible devices from being
> > joined together, and allows some "get info" to report the capability
> > union of all devices if we want to do that.
> 
> I would expect the capability reported per-device via /dev/iommu. 
> Incompatible devices can bind to the same fd but cannot attach to
> the same IOASID. This allows incompatible devices to share locked
> page accounting.

Yeah... I'm not convinced that everything relevant here can be
reported per-device.  I think we may have edge cases where
combinations of devices have restrictions that individual devices in
the set do not.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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