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

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Message-ID: <0a4622ce-3826-4b08-ab81-375887ab6a46@linux.ibm.com>
Date: Mon, 3 Jun 2024 17:50:13 +0200
From: Christian Borntraeger <borntraeger@...ux.ibm.com>
To: Niklas Schnelle <schnelle@...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

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?