linux-kernel - RE: [RFC PATCH 3/3] vfio-pci: Allow to mmap MSI-X table if EEH is supported

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Date:	Thu, 17 Dec 2015 10:08:04 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Alex Williamson' <alex.williamson@...hat.com>,
	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-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
CC:	"nikunj@...ux.vnet.ibm.com" <nikunj@...ux.vnet.ibm.com>,
	"zhong@...ux.vnet.ibm.com" <zhong@...ux.vnet.ibm.com>,
	"aik@...abs.ru" <aik@...abs.ru>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"warrier@...ux.vnet.ibm.com" <warrier@...ux.vnet.ibm.com>
Subject: RE: [RFC PATCH 3/3] vfio-pci: Allow to mmap MSI-X table if EEH is
 supported

> The MSI-X table is paravirtualized on vfio in general and interrupt
> remapping theoretically protects against errant interrupts, so why is
> this PPC64 specific? We have the same safeguards on x86 if we want to
> decide they're sufficient. Offhand, the only way I can think that a
> device can touch the MSI-X table is via backdoors or p2p DMA with
> another device.

Is this all related to the statements in the PCI(e) spec that the
MSI-X table and Pending bit array should in their own BARs?
(ISTR it even suggests a BAR each.)

Since the MSI-X table exists in device memory/registers there is
nothing to stop the device modifying the table contents (or even
ignoring the contents and writing address+data pairs that are known
to reference the CPUs MSI-X interrupt generation logic).

We've an fpga based PCIe slave that has some additional PCIe slaves
(associated with the interrupt generation logic) that are currently
next to the PBA (which is 8k from the MSI-X table).
If we can't map the PBA we can't actually raise any interrupts.
The same would be true if page size is 64k and mapping the MSI-X
table banned.

Do we need to change our PCIe slave address map so we don't need
to access anything in the same page (which might be 64k were we to
target large ppc - which we don't at the moment) as both the
MSI-X table and the PBA?

I'd also note that being able to read the MSI-X table is a useful
diagnostic that the relevant interrupts are enabled properly.

	David