linux-kernel - Re: [RFC PATCH] uio: uio_pci_generic: Add support for MSI interrupts

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Date:	Thu, 4 Jul 2013 17:34:12 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Hans J. Koch" <hjk@...sjkoch.de>, Rob Landley <rob@...dley.net>,
	linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH] uio: uio_pci_generic: Add support for MSI interrupts

On Thu, Jul 04, 2013 at 07:25:23AM -0700, Guenter Roeck wrote:
> On Thu, Jul 04, 2013 at 10:20:23AM +0300, Michael S. Tsirkin wrote:
> > On Thu, Jun 27, 2013 at 10:00:52AM -0700, Guenter Roeck wrote:
> > > On Thu, Jun 27, 2013 at 10:45:01AM +0300, Michael S. Tsirkin wrote:
> > > > On Wed, Jun 26, 2013 at 03:30:23PM -0700, Guenter Roeck wrote:
> > > > > Enable support for MSI interrupts if the device supports it.
> > > > > Since MSI interrupts are edge triggered, it is no longer necessary to
> > > > > disable interrupts in the kernel and re-enable them from user-space.
> > > > > Instead, clearing the interrupt condition in the user space application
> > > > > automatically re-enables the interrupt.
> > > > > 
> > > > > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > > > > ---
> > > > > An open question is if we can just do this unconditionally
> > > > > or if there should be some flag to enable it. A module parameter, maybe ?
> > > > 
> > > > NACK
> > > > 
> > > > UIO is for devices that don't do memory writes.
> > > > Anything that can do writes must be protected by an IOMMU
> > > > and/or have a secure kernel driver, not a UIO stub.
> > > > 
> > > > MSI is done by memory writes so if userspace
> > > > controls the device it can trick it to write
> > > > anywhere in memory.
> > > > 
> > > Just out of curiosity: Since MSI support is mandatory for all PCIE devices,
> > > isn't that possible anyway, even if MSI is not enabled by the kernel ?
> > > All one would need to do is to enable MSI from user space; after all,
> > > the chip configuration space is writable.
> > > 
> > > Thanks,
> > > Guenter
> > 
> > If a device has capability to do writes, sure. So don't do this then :)
> > 
> Not an option. I need to use MSI.
> 
> Not that it matters anymore - turns out it was better writing a specific driver
> for my devices anyway; I needed to be able to disable chip interrupts before
> unloading the driver. But why is it then a reason to NACK this patch ?

There seem to be two cases - either you can't access the device -
and the uio driver is not useful - or you can, and it's not safe.
In both cases the patch does not seem to bring about anything
except user confusion ...

> Besides, doesn't one have to be root anyway to perform such activities,
> which could then be more easily accomplished by writing into /dev/mem ?
> 
> Thanks,
> Guenter

root might not be able to write into /dev/mem.


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MST
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