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: <569F5636.30701@linux.vnet.ibm.com>
Date:	Wed, 20 Jan 2016 17:41:10 +0800
From:	Yongji Xie <xyjxie@...ux.vnet.ibm.com>
To:	David Laight <David.Laight@...LAB.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

On 2016/1/16 1:24, David Laight wrote:
> 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.

IOMMUs supporting interrupt remapping can prevent this case.

> 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.

If we allow the mapping of the MSI-X table, it seems the guest
kernels of some architectures can write invalid data to MSI-X table
when device drivers initialize MSI-X interrupts.

Regards,
Yongji Xie

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