linux-kernel - Re: [RFT][PATCH v1 6/6] vfio: Replace phys_pfn with phys_page for vfio_pin

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Message-ID: <Yq6/qS+AE1LfO+/q@infradead.org>
Date:   Sat, 18 Jun 2022 23:18:17 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>, kwankhede@...dia.com,
        corbet@....net, hca@...ux.ibm.com, gor@...ux.ibm.com,
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Subject: Re: [RFT][PATCH v1 6/6] vfio: Replace phys_pfn with phys_page for
 vfio_pin_pages()

On Fri, Jun 17, 2022 at 03:06:25PM -0700, Nicolin Chen wrote:
> On Fri, Jun 17, 2022 at 01:54:05AM -0700, Christoph Hellwig wrote:
> > There is a bunch of code an comments in the iommu type1 code that
> > suggest we can pin memory that is not page backed.  
> 
> Would you mind explaining the use case for pinning memory that
> isn't page backed? And do we have such use case so far?

Sorry, I should have deleted that sentence.  I wrote it before spending
some more time to dig through the code and all the locked memory has
page backing.  There just seem to be a lot of checks left inbetween
if a pfn is page backed, mostly due to the pfn based calling convetions.

> I can do that. I tried once, but there were just too much changes
> inside type1 code that felt like a chain reaction. If we plan to
> eventually replace with IOMMUFD implementations, these changes in
> type1 might not be necessary, I thought.

To make sure we keep full compatibility I suspect the final iommufd
implementation has to be gradutally created from the existing code
anyway.