linux-kernel - RE: [RFC v6 06/10] PCI: Add a new PCI_BUS_FLAGS_MSI

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Date:	Mon, 18 Apr 2016 11:30:07 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Yongji Xie' <xyjxie@...ux.vnet.ibm.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
CC:	"alistair@...ple.id.au" <alistair@...ple.id.au>,
	"nikunj@...ux.vnet.ibm.com" <nikunj@...ux.vnet.ibm.com>,
	"zhong@...ux.vnet.ibm.com" <zhong@...ux.vnet.ibm.com>,
	"eric.auger@...aro.org" <eric.auger@...aro.org>,
	"aik@...abs.ru" <aik@...abs.ru>,
	"joro@...tes.org" <joro@...tes.org>,
	"will.deacon@....com" <will.deacon@....com>,
	"gwshan@...ux.vnet.ibm.com" <gwshan@...ux.vnet.ibm.com>,
	"warrier@...ux.vnet.ibm.com" <warrier@...ux.vnet.ibm.com>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>
Subject: RE: [RFC v6 06/10] PCI: Add a new PCI_BUS_FLAGS_MSI_REMAP flag

From: Yongji Xie
> Sent: 18 April 2016 11:59
> We introduce a new pci_bus_flags, PCI_BUS_FLAGS_MSI_REMAP
> which indicates all devices on the bus are protected by the
> hardware which supports IRQ remapping(intel naming).
> 
> This flag will be used to know whether it's safe to expose
> MSI-X tables of PCI BARs to userspace. Because the capability
> of IRQ remapping can guarantee the PCI device cannot trigger
> MSIs that correspond to interrupt IDs of other devices.

I'm worried that this entire series is going to break drivers
for existing hardware.

I understand some of the reasoning for 'vm pass through' configurations,
but there will be PCIe devices out there that have the MSI-X tables
in the same BAR as other device registers.
If you are lucky nothing else is in the same 4k area, but I wouldn't
assume it.

In any case, if the hardware can't police the card's master transfers
there is nothing to stop a different bus master block on the card
from raising MSI-X interrupts - they are just a PCIe write.
So all you are doing is raising the bar slightly and giving a very false
sense of security.

	David