Re: [PATCH] pci-assign: Do not reset the device unless the kernel supports it

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 2011-06-07 at 10:14 +0200, Jan Kiszka wrote:
> On 2011-06-07 10:06, Avi Kivity wrote:
> > On 06/07/2011 01:04 AM, Jan Kiszka wrote:
> >> On 2011-06-06 23:48, Alex Williamson wrote:
> >> >  On Mon, 2011-06-06 at 23:30 +0200, Jan Kiszka wrote:
> >> >>  From: Jan Kiszka<jan.kiszka@siemens.com>
> >> >>
> >> >>  At least kernels 2.6.38 and 2.6.39 do not properly support issuing a
> >> >>  reset on an assigned device and corrupt its config space. Prevent
> >> >>  this by checking for a host kernel with the required support,
> >> tagged by
> >> >>  the to-be-introduced KVM_CAP_DEVICE_RESET.
> >> >
> >> >  Wouldn't it be easier just to revert ed78661f in 2.6.39 stable?  I
> >> guess
> >> >  we don't have an option to do that for .38 since stable is done there,
> >> >  but there are also some intel-iommu breakages that won't make
> >> stable for
> >> >  that release.  It seems like the userspace invoked reset resolves
> >> known,
> >> >  demonstrable issues of devices continuing to DMA into guest memory
> >> while
> >> >  ed78661f is mostly a theoretical change.
> >>
> >> Easier would be this patch. But I don't mind reverting the problematic
> >> commit in 39, whatever is preferred. We should just resolve the issue
> >> finally.
> > 
> > Kernel problems should be solved in the kernel (with exceptions of
> > course, but don't see the need here).
> 
> Then please file a revert for stable ASAP.

How's this?  For stable only or course.  Thanks,

Alex

Revert "KVM: Save/restore state of assigned PCI device"

From: Alex Williamson <alex.williamson@redhat.com>

This reverts ed78661f2614d3c9f69c23e280db3bafdabdf5bb as it assumes
the saved PCI state will remain valid for the entire length of time
that it is attached to a guest.  This fails when userspace makes use
of the pci-sysfs reset interface, which invalidates the saved device
state, leaving nothing to be restored after the device is reset on
de-assignment.  This leaves the device in an unusable state.

3.0.0 will add an interface for KVM to save the PCI state in a
buffer unaffected by other callers of pci_reset_function(), but the
most appropriate stable fix seems to be reverting this change since
the original assumption about the device saved state persisting is
incorrect.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
---

 virt/kvm/assigned-dev.c |    5 +----
 1 files changed, 1 insertions(+), 4 deletions(-)


diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
index ae72ae6..e3f1235 100644
--- a/virt/kvm/assigned-dev.c
+++ b/virt/kvm/assigned-dev.c
@@ -197,8 +197,7 @@ static void kvm_free_assigned_device(struct kvm *kvm,
 {
 	kvm_free_assigned_irq(kvm, assigned_dev);
 
-	__pci_reset_function(assigned_dev->dev);
-	pci_restore_state(assigned_dev->dev);
+	pci_reset_function(assigned_dev->dev);
 
 	pci_release_regions(assigned_dev->dev);
 	pci_disable_device(assigned_dev->dev);
@@ -515,7 +514,6 @@ static int kvm_vm_ioctl_assign_device(struct kvm *kvm,
 	}
 
 	pci_reset_function(dev);
-	pci_save_state(dev);
 
 	match->assigned_dev_id = assigned_dev->assigned_dev_id;
 	match->host_segnr = assigned_dev->segnr;
@@ -546,7 +544,6 @@ out:
 	mutex_unlock(&kvm->lock);
 	return r;
 out_list_del:
-	pci_restore_state(dev);
 	list_del(&match->list);
 	pci_release_regions(dev);
 out_disable: