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

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Date:	Thu, 27 Jun 2013 10:00:52 -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, 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
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