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

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Message-ID: <YrAK87zjdOqUF6gB@infradead.org>
Date:   Sun, 19 Jun 2022 22:51:47 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Nicolin Chen <nicolinc@...dia.com>, 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 Mon, Jun 20, 2022 at 12:00:46AM -0300, Jason Gunthorpe 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.  
> 
> AFAIK you can.. The whole follow_pte() mechanism allows raw PFNs to be
> loaded into the type1 maps and the pin API will happily return
> them. This happens in almost every qemu scenario because PCI MMIO BAR
> memory ends up routed down this path.

Indeed, my read wasn't deep enough.  Which means that we can't change
the ->pin_pages interface to return a struct pages array, as we don't
have one for those.