linux-kernel - Re: [PATCH V3] VFIO driver: Non-privileged user level PCI drivers

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Message-ID: <20100729142705.GA25879@redhat.com>
Date:	Thu, 29 Jul 2010 17:27:05 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Tom Lyon <pugs@...co.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, randy.dunlap@...cle.com, arnd@...db.de,
	chrisw@...s-sol.org, joro@...tes.org, hjk@...utronix.de,
	avi@...hat.com, gregkh@...e.de, aafabbri@...co.com,
	scofeldm@...co.com
Subject: Re: [PATCH V3] VFIO driver: Non-privileged user level PCI drivers

On Wed, Jul 28, 2010 at 04:13:19PM -0600, Alex Williamson wrote:
> On Thu, 2010-07-29 at 00:57 +0300, Michael S. Tsirkin wrote:
> > On Wed, Jul 28, 2010 at 03:57:02PM -0600, Alex Williamson wrote:
> > > 
> > > Something like GET_MSIX_VECTORS seems like a user library routine to me.
> > > The PCI config space is well specified and if we try to do more than
> > > shortcut trivial operations (like getting the BAR length), we risk
> > > losing functionality.  And for my purposes, translating to and from a
> > > made up API to PCI for the guest seems like a pain.
> > 
> > Won't a userspace library do just as well for you?
> 
> You mean aside from qemu's reluctance to add dependencies for more
> libraries?

Main reason is portability. So as long as it's kvm-only stuff, they
likely won't care.

> My only concern is that I want enough virtualized/raw config
> space that I'm not always translating PCI config accesses from the guest
> into some userspace API.  If it makes sense to do this for things like
> MSI, since I need someone to figure out what resources can actually be
> allocated on the host, then maybe the library makes sense for that.
> Then again, if every user needs to do this, let the vfio kernel driver
> check what it can get and virtualize the available MSIs in exposed
> config space, and my driver would be even happier.
> 
> Alex

It would?  guest driver might or might not work if you reduce the number
of vectors for device.  So I think you need an API to find out whether
all vectors can be allocated.

And these are examples of why virtualizing is wrong:
1. hides real hardware
2. no way to report errors

-- 
MST
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