linux-kernel - Re: [RFC PATCH 0/7] vfio: Add alias region uapi for device feature

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Message-ID: <CALzav=cRjQedF_g7qkJfrAtUEZKsME5=QpC+C6B7-nB_j-jc_Q@mail.gmail.com>
Date: Fri, 3 Oct 2025 14:58:50 -0700
From: David Matlack <dmatlack@...gle.com>
To: Mahmoud Adam <mngyadam@...zon.de>
Cc: kvm@...r.kernel.org, alex.williamson@...hat.com, jgg@...pe.ca, 
	kbusch@...nel.org, benh@...nel.crashing.org, 
	David Woodhouse <dwmw@...zon.co.uk>, pravkmr@...zon.de, nagy@...aternagy.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/7] vfio: Add alias region uapi for device feature

On Wed, Sep 24, 2025 at 7:11 AM Mahmoud Adam <mngyadam@...zon.de> wrote:
>
> This adds two new region flags:
> - VFIO_REGION_INFO_FLAG_ALIAS: set on alias regions.
> - VFIO_REGION_INFO_FLAG_WC: indicates WC is in effect for that region.

Once you settle on a uAPI, this would be a good candidate for some
VFIO selftests coverage [1][2].

Assuming the uAPI supports setting up equivalent aliases of BARs, you
can use the VFIO selftests driver framework to drive some meaningful
usage of the aliases. For example, you could write a test that
overrides bars[n].vaddr in struct vfio_pci_device to force the drivers
to use aliased BARs that you set up and mmapped, and make sure the
device can still perform memcpys as it should afterward.

But regardless, there is still plenty of opportunity for sanity tests
that verify the new ioctls fail and succeed when they should, and that
mmap works as expected.

Meaningfully verifying that VFIO_REGION_INFO_FLAG_WC is being applied
correctly might be a challenge.

[1] https://lore.kernel.org/kvm/20250822212518.4156428-1-dmatlack@google.com/
[2] https://lore.kernel.org/kvm/20250930124221.39523455.alex.williamson@redhat.com/