linux-kernel - Re: [PATCH v3 0/3] vfio/pci: s390: Fix issues preventing VFIO_PCI

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Message-ID: <8891a2fbc3b211ac4c3ab87ecf786214bd9b2a55.camel@linux.ibm.com>
Date: Wed, 05 Jun 2024 09:49:35 +0200
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Christian Borntraeger <borntraeger@...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>,
        Sven Schnelle <svens@...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>
Cc: linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v3 0/3] vfio/pci: s390: Fix issues preventing
 VFIO_PCI_MMAP=y for s390 and enable it

On Tue, 2024-06-04 at 11:27 +0200, Niklas Schnelle wrote:
> 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 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?
> 
> That matches my understanding as well.
> 
> > 
> > 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.
> 

Was a bit unclear/wrong above. Obviously MMIO mappings can't be
"swapped out" I should have said "are subject to page faults".