Re: [Qemu-devel] [Xen-devel] [iGVT-g] [vfio-users] [PATCH v3 00/11] igd passthrough chipset tweaks

[David Woodhouse <dwmw2@infradead.org>]

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On Tue, 2016-02-02 at 07:54 -0700, Alex Williamson wrote:
> 
> I first glance I like it, but there's a problem, it assumes there is a
> host driver for the device that will permanently release the device from
> the RMRR even after the device is unbound.  Currently we don't have a
> requirement that the user must first bind the device to a native host
> driver, unbind it, and only then is it eligible for device assignment.

It doesn't *have* to be a full native driver. It can be a PCI quirk
(the USB controllers could potentially do it that way, although they
don't). Or a stub 'shut it down' driver, potentially even done somehow
through VFIO.

But fundamentally, in all of these cases you have to do *something* to
stop the BIOS-controlled DMA. Otherwise the RMRR shouldn't have been
there in the first place, surely?

But for the gfx case... what *do* we have to do? Does the VMM (and the
VM's BIOS, between them) have to provision a "stolen" region of guest
memory and point the gfx framebuffer at that?

Once we have a proper handle on precisely what needs to happen, we can
have a better conversation about where/how to do that...

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation


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