linux-kernel - Re: [PATCH v6 0/3] vfio/pci: s390: Fix issues preventing VFIO_PCI

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Message-ID: <20250225203524.GA516498@bhelgaas>
Date: Tue, 25 Feb 2025 14:35:24 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Niklas Schnelle <schnelle@...ux.ibm.com>
Cc: Christoph Hellwig <hch@....de>,
	Alexandra Winter <wintera@...ux.ibm.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Gerd Bayer <gbayer@...ux.ibm.com>,
	Matthew Rosato <mjrosato@...ux.ibm.com>,
	Jason Gunthorpe <jgg@...pe.ca>,
	Thorsten Winkler <twinkler@...ux.ibm.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Julian Ruess <julianr@...ux.ibm.com>,
	Halil Pasic <pasic@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v6 0/3] vfio/pci: s390: Fix issues preventing
 VFIO_PCI_MMAP=y for s390 and enable it

On Tue, Feb 25, 2025 at 09:59:13AM +0100, Niklas Schnelle wrote:
> On Mon, 2025-02-24 at 14:53 -0600, Bjorn Helgaas wrote:
> > On Fri, Feb 14, 2025 at 02:10:51PM +0100, Niklas Schnelle wrote:
> > > With the introduction of memory I/O (MIO) instructions enbaled in commit
> > > 71ba41c9b1d9 ("s390/pci: provide support for MIO instructions") s390
> > > gained support for direct user-space access to mapped PCI resources.
> > > Even without those however user-space can access mapped PCI resources
> > > via the s390 specific MMIO syscalls. There is thus nothing fundamentally
> > > preventing s390 from supporting VFIO_PCI_MMAP, allowing user-space
> > > drivers to access PCI resources without going through the pread()
> > > interface. To actually enable VFIO_PCI_MMAP a few issues need fixing
> > > however.
> > > 
> > > Firstly the s390 MMIO syscalls do not cause a page fault when
> > > follow_pte() fails due to the page not being present. This breaks
> > > vfio-pci's mmap() handling which lazily maps on first access.
> > > 
> > > Secondly on s390 there is a virtual PCI device called ISM which has
> > > a few oddities. For one it claims to have a 256 TiB PCI BAR (not a typo)
> > > which leads to any attempt to mmap() it fail with the following message:
> > > 
> > >     vmap allocation for size 281474976714752 failed: use vmalloc=<size> to increase size
> > > 
> > > Even if one tried to map this BAR only partially the mapping would not
> > > be usable on systems with MIO support enabled. So just block mapping
> > > BARs which don't fit between IOREMAP_START and IOREMAP_END. Solve this
> > > by keeping the vfio-pci mmap() blocking behavior around for this
> > > specific device via a PCI quirk and new pdev->non_mappable_bars
> > > flag.
> > > 
> > > As noted by Alex Williamson With mmap() enabled in vfio-pci it makes
> > > sense to also enable HAVE_PCI_MMAP with the same restriction for pdev->
> > > non_mappable_bars. So this is added in patch 3 and I tested this with
> > > another small test program.
> > > 
> > > Note:
> > > For your convenience the code is also available in the tagged
> > > b4/vfio_pci_mmap branch on my git.kernel.org site below:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/niks/linux.git/
> > > 
> > > Thanks,
> > > Niklas
> > > 
> > > Link: https://lore.kernel.org/all/c5ba134a1d4f4465b5956027e6a4ea6f6beff969.camel@linux.ibm.com/
> > > Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> > > ---
> > > Changes in v6:
> > > - Add a patch to also enable PCI resource mmap() via sysfs and proc
> > >   exlcluding pdev->non_mappable_bars devices (Alex Williamson)
> > > - Added Acks
> > > - Link to v5: https://lore.kernel.org/r/20250212-vfio_pci_mmap-v5-0-633ca5e056da@linux.ibm.com
> > 
> > I think the series would be more readable if patch 2/3 included all
> > the core changes (adding pci_dev.non_mappable_bars, the 3/3
> > pci-sysfs.c and proc.c changes to test it, and I suppose the similar
> > vfio_pci_core.c change), and we moved all the s390 content from 2/3 to
> > 3/3.
> 
> Maybe we could do the following:
> 
> 1/3: As is
> 
> 2/3: Introduces pdev->non_mappable_bars and the checks in vfio and
> proc.c/pci-sysfs.c. To make the flag handle the vfio case with
> VFIO_PCI_MMAP gone, a one-line change in s390 will set pdev-
> >non_mappable_bars = 1 for all PCI devices.

What if you moved the vfio_pci_core.c change to patch 3?  Then I think
patch 2 would do nothing at all (since there's nothing that sets
non_mappable_bars), and all the s390 stuff would be in patch 3?

Not sure if that's possible, but I think it's a little confusing to
have the s390 changes split across patch 2 and 3.

> 3/3: Changes setting pdev->non_mappable_bars = 1 in s390 to only the
> ISM device using the quirk handling and adds HAVE_PCI_MMAP.
> 
> Thanks,
> Niklas