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

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Message-ID: <20151008170121.GA2013@scylladb.com>
Date:	Thu, 8 Oct 2015 20:01:21 +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 07:43:04PM +0300, Michael S. Tsirkin wrote:
> On Thu, Oct 08, 2015 at 04:28:34PM +0300, Gleb Natapov wrote:
> > On Thu, Oct 08, 2015 at 04:20:04PM +0300, Michael S. Tsirkin wrote:
> > > On Thu, Oct 08, 2015 at 03:27:37PM +0300, Gleb Natapov wrote:
> > > > 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.
> > > 
> > > It isn't worse. It's a sane interface. Whoever enables bus mastering
> > > must be careful.  If userspace enables bus mastering then userspace
> > > needs to be very careful with the device to avoid corrupting kernel
> > > memory.  If kernel does it, it's kernel's responsibility.
> > > 
> > Although this definition of sanity sounds strange to me, but lets
> > flow with it for the sake of this email: would it be OK if proposed
> > interface refused to work if bus mastering is not already enabled by
> > userspace?
> 
> An interface could be acceptable if there's a fallback where it
> works without BM but slower (e.g. poll pending bits).
> 
OK.

> But not the proposed one.
>
Why? Greg is against ioctl interface so it will be reworked, by besides
that what is wrong with the concept of binding msi-x interrupt to
eventfd?
 
> Really, there's more to making msi-x work with
> userspace drivers than this patch. As I keep telling people, you would
> basically reimplement vfio/pci. Go over it, and see for yourself.
> Almost everything it does is relevant for msi-x.  It's just wrong to
> duplicate so much code.
> 
The patch is tested and works with msi-x. Restricting access to msi-x
registers that vfio does is not relevant here.

--
			Gleb.
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