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Date: Thu, 6 Jun 2024 11:27:18 -0600
From: Alex Williamson <alex.williamson@redhat.com>
To: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Niklas Schnelle <schnelle@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>, Gerd Bayer <gbayer@linux.ibm.com>,
 Matthew Rosato <mjrosato@linux.ibm.com>, Jason Gunthorpe <jgg@ziepe.ca>,
 linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
 kvm@vger.kernel.org
Subject: Re: [PATCH v3 0/3] vfio/pci: s390: Fix issues preventing
 VFIO_PCI_MMAP=y for s390 and enable it
Message-ID: <20240606112718.0171f5b3.alex.williamson@redhat.com>
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, 3 Jun 2024 17:50:13 +0200
Christian Borntraeger <borntraeger@linux.ibm.com> wrote:

> Am 29.05.24 um 13:36 schrieb Niklas Schnelle:
> > 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.
> > 
> > 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/  
> 
> 
> I guess its now mostly a question of who picks those patches? Alex?
> 
> Any patch suitable for stable?

Nothing here looks like stable material to me.  1/ only becomes an
issue when mmap of MMIO is allowed on s390 (ie. 3/), 2/ is generic, but
only really targets a device found on s390, and finally 3/ is
essentially enabling a new feature.

If we expect any conflicts with 1/ in the next merge window I can take
a branch for it and apply 2/ and 3/ through the vfio tree, otherwise I
can bring them all through the vfio tree if the s390 folks agree.
Thanks,

Alex