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

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Message-ID: <20130704142523.GA22081@roeck-us.net>
Date:	Thu, 4 Jul 2013 07:25:23 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	"Michael S. Tsirkin" <mst@...hat.com>
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 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 ?
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
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