linux-kernel - Re: [RFC] Making vma_to_pfn() public (due to vm

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Message-ID: <20251027201711.65e82a4f@shazbot.org>
Date: Mon, 27 Oct 2025 20:17:11 -0600
From: Alex Williamson <alex@...zbot.org>
To: Mukesh R <mrathor@...ux.microsoft.com>
Cc: alex.williamson@...hat.com, joe@...ches.com, kvm@...r.kernel.org,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "wei.liu@...nel.org" <wei.liu@...nel.org>
Subject: Re: [RFC] Making vma_to_pfn() public (due to vm_pgoff change)

On Mon, 27 Oct 2025 14:21:56 -0700
Mukesh R <mrathor@...ux.microsoft.com> wrote:

> Hi Alex,
> 
> This regards vfio passthru support on hyperv running linux as dom0 aka
> root. At a high level, cloud hypervisor uses vfio for set up as usual,
> then maps the mmio ranges via the hyperv linux driver ioctls.
> 
> Over a year ago, when working on this I had used vm_pgoff to get the pfn
> for the mmio, that was 5.15 and early 6.x kernels. Now that I am porting
> to 6.18 for upstreaming, I noticed:
> 
> commit aac6db75a9fc
> Author: Alex Williamson <alex.williamson@...hat.com>
>     vfio/pci: Use unmap_mapping_range()
> 
> changed the behavior and vm_pgoff is no longer holding the pfn. In light
> of that, I wondered if the following minor change, making vma_to_pfn() 
> public (after renaming it), would be acceptable to you.

How do you know the device is using vfio_pci_core_mmap() with these
semantics for vm_pgoff versus something like nvgrace_gpu_mmap() that
uses vm_pgoff more like you're expecting?  vma_to_pfn() is specific to
the vfio-pci-core semantics, it's not portable to expose for other use
cases.  Thanks,

Alex