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Re: [Qemu-devel] [RFC PATCH] vfio: VFIO Driver core framework

From: Bhushan Bharat-R65777
Subject: Re: [Qemu-devel] [RFC PATCH] vfio: VFIO Driver core framework
Date: Fri, 2 Dec 2011 18:11:27 +0000

> -----Original Message-----
> From: address@hidden [mailto:address@hidden On
> Behalf Of Stuart Yoder
> Sent: Friday, December 02, 2011 8:11 PM
> To: Alex Williamson
> Cc: Alexey Kardashevskiy; address@hidden; address@hidden;
> address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden; address@hidden; address@hidden
> sol.org; Yoder Stuart-B08248; address@hidden;
> address@hidden; address@hidden; Wood Scott-B07421;
> address@hidden
> Subject: Re: [Qemu-devel] [RFC PATCH] vfio: VFIO Driver core framework
> On Thu, Dec 1, 2011 at 3:25 PM, Alex Williamson
> <address@hidden> wrote:
> > On Thu, 2011-12-01 at 14:58 -0600, Stuart Yoder wrote:
> >> One other mechanism we need as well is the ability to enable/disable
> >> a domain.
> >>
> >> For example-- suppose a device is assigned to a VM, the device is in
> >> use when the VM is abruptly terminated.  The VM terminate would shut
> >> off DMA at the IOMMU, but now the device is in an indeterminate
> >> state.   Some devices have no simple reset bit and getting the device
> >> back into a sane state could be complicated-- something the
> >> hypervisor doesn't want to do.
> >>
> >> So now KVM restarts the VM, vfio init happens for the device and  the
> >> IOMMU for that device is re-configured, etc, but we really can't
> >> re-enable DMA until the guest OS tells us (via an hcall) that it is
> >> ready.   The guest needs to get the assigned device in a sane state
> >> before DMA is enabled.
> >
> > Giant red flag.  We need to paravirtualize the guest?  Not on x86.
> It's the reality we have to deal with, but doing this would obviously
> only apply to platforms that need it.
> > Some
> > devices are better for assignment than others.  PCI devices are moving
> > towards supporting standard reset mechanisms.
> >
> >> Does this warrant a new domain enable/disable API, or should we make
> >> this part of the setup API we are discussing here?
> >
> > What's wrong with simply not adding any DMA mapping entries until you
> > think your guest is ready?  Isn't that effectively the same thing?
> > Unmap ~= disable.  If the IOMMU API had a mechanism to toggle the
> > iommu domain on and off, I wouldn't be opposed to adding an ioctl to
> > do it, but it really seems like just a shortcut vs map/unmap.  Thanks,
> Yes, we could do something like that I guess.

How do we determine whether guest is ready or not? There can be multiple device 
get ready at different time.
Further if guest have given the device to it user level process or its guest. 
Should not there be some mechanism for a guest to enable/disable on per device 
or group?


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