linux-kernel - Re: [PATCH] uio/uio_pci_generic: Add SR-IOV support

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Message-ID: <1506971418.18322.47.camel@infradead.org>
Date:   Mon, 02 Oct 2017 20:10:18 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Don Dutile <ddutile@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "Bryant G. Ly" <bryantly@...ux.vnet.ibm.com>,
        Bodong Wang <bodong@...lanox.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH] uio/uio_pci_generic: Add SR-IOV support

On Mon, 2017-10-02 at 14:52 -0400, Don Dutile wrote:
> On 10/02/2017 08:35 AM, David Woodhouse wrote:
> > This would allow you to enable SR-IOV on a PF before its driver is
> > loaded, right? Even when that driver *is* going to need to perform
> > resource management for those VFs?
> > 
> > Would existing drivers cope with SR-IOV being enabled, and VFs being
> > assigned to guests, before they're loaded? If so then sure, let's do it
> > generically. But I'm not sure that's the case.
> > 
> No better than a uio driver/mgmt api that may have to configure a PF
> before a VF is enabled.

Conceptually, the current model is that you don't have SR-IOV until you
have a driver loaded for the physical function which can do any
necessary resource management.

That's *why* the generic "sriov_numvfs" interface in sysfs isn't
present until such a driver is loaded.

In the UIO case, *userspace* is responsible for the PF. So it's not an
"attack vector"; we let userspace do what it likes with the PF and that
includes enabling SR-IOV too.

Do we actually *disable* SR-IOV when a (UIO or in-kernel) driver for
the PF is unloaded? If not, that's the only "attack vector" I see — to
load a driver which permits SR-IOV to be enabled, and do so, and then
unload it and load a different driver which doesn't cope.

And each driver in that scenario can be either an in-kernel driver or
UIO+userspace; it doesn't matter either way. The patch I sent is just
following the *existing* model.

But sure, my question was intended to ask whether we want to *stick*
with that model. Given the answers I got, my own conclusion was that we
probably do...

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