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Message-ID: <b816efd7dd3d5f6fead3433d8a800c7f4372a091.camel@linux.ibm.com>
Subject: Re: [PATCH v3 0/3] vfio/pci: s390: Fix issues preventing
 VFIO_PCI_MMAP=y for s390 and enable it
From: Niklas Schnelle <schnelle@linux.ibm.com>
To: Christian Borntraeger <borntraeger@linux.ibm.com>,
        Gerald Schaefer
 <gerald.schaefer@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>, Vasily
 Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Alex Williamson
 <alex.williamson@redhat.com>,
        Gerd Bayer <gbayer@linux.ibm.com>,
        Matthew
 Rosato <mjrosato@linux.ibm.com>,
        Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Date: Tue, 04 Jun 2024 11:27:53 +0200
In-Reply-To: <0a4622ce-3826-4b08-ab81-375887ab6a46@linux.ibm.com>
References: <20240529-vfio_pci_mmap-v3-0-cd217d019218@linux.ibm.com>
	 <0a4622ce-3826-4b08-ab81-375887ab6a46@linux.ibm.com>
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On Mon, 2024-06-03 at 17:50 +0200, Christian Borntraeger wrote:
> Am 29.05.24 um 13:36 schrieb Niklas Schnelle:
> > With the introduction of memory I/O (MIO) instructions enbaled in commi=
t
> > 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 fundamentall=
y
> > preventing s390 from supporting VFIO_PCI_MMAP allowing user-space drive=
rs
> > to access PCI resources without going through the pread() interface.
> > To actually enable VFIO_PCI_MMAP a few issues need fixing however.
> >=20
> > 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.
> >=20
> > 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=
:
> >=20
> >      vmap allocation for size 281474976714752 failed: use vmalloc=3D<si=
ze> to increase size
> >=20
> > 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.
> >=20
> > 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/
>=20
>=20
> I guess its now mostly a question of who picks those patches? Alex?

That matches my understanding as well.

>=20
> Any patch suitable for stable?

I'd almost say all but the last one may be candidates for stable. I
found it hard to pinpoint a specific commit they fix though, hence the
lack of Fixes tag. For the first one I'm actually not sure if e.g.
rdma-core users could also run into this problem when they get swapped
out as I'm not sure if the mapping is pinned there.