linux-kernel - Re: [PATCH 2/2] uio: new driver to support PCI MSI-X

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Date:	Tue, 6 Oct 2015 00:54:55 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	hjk@...sjkoch.de, gregkh@...ux-foundation.org,
	linux-kernel@...r.kernel.org, dev@...k.org
Subject: Re: [PATCH 2/2] uio: new driver to support PCI MSI-X

On Thu, Oct 01, 2015 at 11:33:06AM +0300, Michael S. Tsirkin wrote:
> Just forwarding events is not enough to make a valid driver.
> What is missing is a way to access the device in a safe way.

Thinking about it some more, maybe some devices don't do DMA, and merely
signal events with MSI/MSI-X.

The fact you mention igb_uio in the cover letter seems to hint that this
isn't the case, and that the real intent is to abuse it for DMA-capable
devices, but still ...

If we assume such a simple device, we need to block userspace from
tweaking at least the MSI control and the MSI-X table.
And changing BARs might make someone else corrupt the MSI-X
table, so we need to block it from changing BARs, too.

Things like device reset will clear the table.  I guess this means we
need to track access to reset, too, make sure we restore the
table to a sane config.

PM  capability can be used to reset things tooI think. Better be
careful about that.

And a bunch of devices could be doing weird things that need
to be special-cased.

All of this is what VFIO is already dealing with.

Maybe extending VFIO for this usecase, or finding another way to share
code might be a better idea than duplicating the code within uio?

-- 
MST
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