linux-kernel - Re: [RFC 00/18] Pkernfs: Support persistence for live update

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Message-ID: <20240207152854.GL31743@ziepe.ca>
Date: Wed, 7 Feb 2024 11:28:54 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Gowans, James" <jgowans@...zon.com>
Cc: "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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Subject: Re: [RFC 00/18] Pkernfs: Support persistence for live update

On Wed, Feb 07, 2024 at 02:56:33PM +0000, Gowans, James wrote:
> 2. Tell VFIO to avoid mapping the memory in again after live update
> because it already exists.
> https://github.com/jgowans/qemu/commit/6e4f17f703eaf2a6f1e4cb2576d61683eaee02b0
> (the above flag should only be set *after* live update...).

Definately no to that entire idea. It completely breaks how the memory
lifetime model works in iommufd.

iommufd has to re-establish its pins, and has to rebuild all its
mapping data structures. Otherwise it won't work correctly at all.

This is what I was saying in the other thread, you can't just ignore
fully restoring the iommu environment.

The end goal must be to have fully reconstituted iommufd with all its
maps, ioas's, and memory pins back to fully normal operation.

IMHO you need to focus on atomic replace where you go from the frozen
pkernfs environment to a live operating enviornment by hitlessly
replacing the IO page table in the HW. Ie going from an
IOMMU_DOMAIN_PKERFS to an IOMMU_DOMAIN_PAGING owned by iommufd that
describes exactly the same translation.

"adopting" an entire io page table with unknown contents, and still
being able to correctly do map/unmap seems way too hard.

Jason