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Date: Fri, 10 Dec 2021 08:30:28 -0400
From: Jason Gunthorpe <jgg@nvidia.com>
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()
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To: "Tian, Kevin" <kevin.tian@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, "Jiang, Dave" <dave.jiang@intel.com>, Logan Gunthorpe <logang@deltatee.com>, LKML <linux-kernel@vger.kernel.org>, Bjorn Helgaas <helgaas@kernel.org>, Marc Zygnier <maz@kernel.org>, Alex Williamson <alex.williamson@redhat.com>, "Dey, Megha" <megha.dey@intel.com>, "Raj, Ashok" <ashok.raj@intel.com>, "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jon Mason <jdmason@kudzu.us>, Allen Hubbe <allenbh@gmail.com>, "linux-ntb@googlegroups.com" <linux-ntb@googlegroups.com>, "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>, Heiko Carstens <hca@linux.ibm.com>, Christian Borntraeger <borntraeger@de.ibm.com>, "x86@kernel.org" <x86@kernel.org>, Joerg Roedel <jroedel@suse.de>, "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>
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On Fri, Dec 10, 2021 at 07:36:12AM +0000, Tian, Kevin wrote:

> /*
>  * The MSIX mappable capability informs that MSIX data of a BAR can be mmapped
>  * which allows direct access to non-MSIX registers which happened to be within
>  * the same system page.
>  *
>  * Even though the userspace gets direct access to the MSIX data, the existing
>  * VFIO_DEVICE_SET_IRQS interface must still be used for MSIX configuration.
     ^^^^^^^^^^^^^^^^^^^^^

It is this I think we don't want, there should be no hypervisor
involvment on installing the addr/data pair

Guessing by what I saw in PPC there is a hypercall to install a MSI
and the guest never touches the page even though it is
mapped. Presumably there is some IR like thing making this secured

PPC has the same basic problem, I think they have a hypercall to
install a MSI, not a hypercall to provision a generic addr/data pair.

Jason