linux-kernel - RE: [RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate filtering of MSIs is supported

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CCC6833@AcuExch.aculab.com>
Date:	Fri, 15 Jan 2016 17:24:22 +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>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
CC:	"nikunj@...ux.vnet.ibm.com" <nikunj@...ux.vnet.ibm.com>,
	"zhong@...ux.vnet.ibm.com" <zhong@...ux.vnet.ibm.com>,
	"corbet@....net" <corbet@....net>, "aik@...abs.ru" <aik@...abs.ru>,
	"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 PATCH v3 3/5] PCI: Add host bridge attribute to indicate
 filtering of MSIs is supported

From: Yongji Xie
> Sent: 15 January 2016 07:06
>
> MSI-X tables are not allowed to be mmapped in vfio-pci
> driver in case that user get to touch this directly.
> This will cause some performance issues when when PCI
> adapters have critical registers in the same page as
> the MSI-X table.
...
If the driver wants to generate an incorrect MSI-X interrupt
it can do so by requesting the device do a normal memory transfer
to the target address area that raises MSI-X interrupts.
So disabling writes to the MSI-X table (and pending bit array)
areas only raises the bar very slightly.
A device may also give the driver write access to the MSI-X
table through other addresses.

This seems to make disallowing the mapping of the MSI-X table
rather pointless.

I've also dumped out the MSI-X table (during development) to
check that the values are being written there correctly.

	David