linux-kernel - Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

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Message-ID: <20151008122737.GI11716@scylladb.com>
Date:	Thu, 8 Oct 2015 15:27:37 +0300
From:	Gleb Natapov <gleb@...lladb.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Avi Kivity <avi@...lladb.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Vlad Zolotarov <vladz@...udius-systems.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, hjk@...sjkoch.de, corbet@....net,
	bruce.richardson@...el.com, avi@...udius-systems.com,
	gleb@...udius-systems.com, stephen@...workplumber.org,
	alexander.duyck@...il.com
Subject: Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

On Thu, Oct 08, 2015 at 03:06:07PM +0300, Michael S. Tsirkin wrote:
> On Thu, Oct 08, 2015 at 12:44:09PM +0300, Avi Kivity wrote:
> > 
> > 
> > On 10/08/2015 12:16 PM, Michael S. Tsirkin wrote:
> > >On Thu, Oct 08, 2015 at 11:46:30AM +0300, Avi Kivity wrote:
> > >>
> > >>On 10/08/2015 10:32 AM, Michael S. Tsirkin wrote:
> > >>>On Thu, Oct 08, 2015 at 08:33:45AM +0300, Avi Kivity wrote:
> > >>>>It is good practice to defend against root oopsing the kernel, but in some
> > >>>>cases it cannot be achieved.
> > >>>Absolutely. That's one of the issues with these patches. They don't even
> > >>>try where it's absolutely possible.
> > >>>
> > >>Are you referring to blocking the maps of the msix BAR areas?
> > >For example. There are more. I listed some of the issues on the mailing
> > >list, and I might have missed some.  VFIO has code to address all this,
> > >people should share code to avoid duplication, or at least read it
> > >to understand the issues.
> > 
> > All but one of those are unrelated to the patch that adds msix support.
> 
> They are related because msix support enables bus mastering.  Without it
> device is passive and can't harm anyone. With it, suddently you need to
> be very careful with the device to avoid corrupting kernel memory.
> 
Most (if not all) uio_pci_generic users enable pci bus mastering. The
fact that they do that without even tainting the kernel like the patch
does make current situation much worse that with the patch.

> > I can't comment on iommu overhead; for my use case it is likely negligible
> > and we will use an iommu when available; but apparently it matters for
> > others.
> 
> You and Vlad are the only ones who brought this up.
> So maybe you should not bring it up anymore.
> 
Common, you were CCed to at least this one:

 We have a solution that makes use of IOMMU support with vfio.  The 
 problem is there are multiple cases where that support is either not 
 available, or using the IOMMU provides excess overhead.


http://dpdk.org/ml/archives/dev/2015-October/024560.html

--
			Gleb.
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