Re: Hard and silent lock up since linux 3.14 with PCIe pass through (vfio)

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

On Wed, 2014-10-29 at 18:57 +0100, Andreas Hartmann wrote:
> Alex Williamson schrieb:
> > On Wed, 2014-10-29 at 17:47 +0100, Andreas Hartmann wrote:
> >> Alex Williamson wrote:
> >>> On Sat, 2014-10-25 at 08:03 +0200, Andreas Hartmann wrote:
> >>>>
> >>>> Out of interest:
> >>>> Bjorn's patch disables vc save/restore support - and the machine works
> >>>> fine again. Why is it needed at all if it seems to work perfectly w/o
> >>>> it? What's the additional benefit? Or in other words: What am I missing
> >>>> until today :-) ? What would be better? What could I do more?
> >>>
> >>>
> >>> You're right, in the configuration you have the endpoint device has a
> >>> Virtual Channel capability but the upstream root port does not.  The
> >>> spec is not at all clear about defining the endpoints for enabling
> >>> Virtual Channel in each type of configuration, but I think that if we
> >>> have an upstream port that does not support Virtual Channel, we can skip
> >>> the save/restore.  Please test the patch below.
> >>>
> >>> I'm also still completely confused about whether this is a VC
> >>> save/restore issue or a bus reset issue.  You originally bisected this
> >>> back to the VC save/restore patch, but you also found that a manual,
> >>> setpci-based bus reset triggered a system hang.
> >>
> >> With your additional patch posted here:
> >> http://article.gmane.org/gmane.linux.kernel.pci/36162
> > 
> > Right, a reset via sysfs also triggered it with that patch, but the
> > reset via setpci is independent of any VC save/restore and still hung
> > your box.
> > 
> >>
> >>>  I believe that
> >>> re-ordering the kernel reset mechanisms also triggered this.  Since
> >>> recent versions of QEMU are going to favor a bus reset over PM reset, I
> >>> don't have a lot of confidence that we're actually solving the problem
> >>> for you.  Please make sure to test with a recent QEMU to be sure we'll
> >>> do a bus reset.
> >>
> >> I'm running qemu 2.1.0 (newest is 2.1.2 - but this shouldn't be a
> >> problem) and tested w/ linux 3.17.
> > 
> > Yep, just want to make sure it's QEMU new enough to do a bus reset and
> > kernel with matching support.
> > 
> >>> diff --git a/drivers/pci/vc.c b/drivers/pci/vc.c
> >>> index 7e1304d..6d13d34 100644
> >>> --- a/drivers/pci/vc.c
> >>> +++ b/drivers/pci/vc.c
> >>> @@ -339,6 +339,25 @@ static int pci_vc_do_save_buffer(struct pci_dev *dev, int pos,
> >>>  	return buf ? 0 : len;
> >>>  }
> >>>  
> >>> +/**
> >>> + * pci_vc_needs_save - Determine whether a VC capability needs to be saved
> >>> + * @dev: device
> >>> + * @id: VC capability ID (VC/VC9/MFVC)
> >>> + *
> >>> + * In configurations where we have a VC or MFVC capability, but the upstream
> >>> + * device does not, we assume that VC save (and therefore restore) is not
> >>> + * necessary.  The intention is to only do VC save/restore in configuration
> >>> + * where it's necessary and hopefully avoid reset issues.
> >>> + */
> >>> +static bool pci_vc_needs_save(struct pci_dev *dev, u16 id)
> >>> +{
> >>> +	if (id == PCI_EXT_CAP_ID_VC9 || pci_is_root_bus(dev->bus) ||
> >>> +	    pci_find_ext_capability(dev->bus->self, PCI_EXT_CAP_ID_VC))
> >>> +		return true;
> >>> +
> >>> +	return false;
> >>> +}
> >>> +
> >>>  static struct {
> >>>  	u16 id;
> >>>  	const char *name;
> >>> @@ -362,7 +381,7 @@ int pci_save_vc_state(struct pci_dev *dev)
> >>>  		struct pci_cap_saved_state *save_state;
> >>>  
> >>>  		pos = pci_find_ext_capability(dev, vc_caps[i].id);
> >>> -		if (!pos)
> >>> +		if (!posi || !pci_vc_needs_save(dev, vc_caps[i].id))
> >>                         ^
> >> This should be most probably !pos (and not !posi - because !posi does
> >> through a compile error).
> > 
> > Oops, sorry.
> > 
> >>>  			continue;
> >>>  
> >>>  		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> >>> @@ -422,7 +441,7 @@ void pci_allocate_vc_save_buffers(struct pci_dev *dev)
> >>>  	for (i = 0; i < ARRAY_SIZE(vc_caps); i++) {
> >>>  		int len, pos = pci_find_ext_capability(dev, vc_caps[i].id);
> >>>  
> >>> -		if (!pos)
> >>> +		if (!pos || !pci_vc_needs_save(dev, vc_caps[i].id))
> >>>  			continue;
> >>>  
> >>>  		len = pci_vc_do_save_buffer(dev, pos, NULL, false);
> >>
> >> W/ the above patch, the machine hangs again (w/ qemu and setpci), but w/
> >> Bjorn's patch (and nothing more applied) which disables vc save/restore,
> >> the machine just works fine ... . I especially retested this case to be
> >> really sure. I'm so sorry. But that's how it behaves here :-(
> > 
> > Hmm, the intention was that this should effectively do the same thing as
> > Bjorn's patch.  The Atheros device (03:00.0) reports a VC capability but
> > the root port above it (00:05.0) does not.
> 
> Are you sure, that this patch really works (-> here!) as expected? Would
> it be possible to add some debug output printing to the actual console
> (not to log file) to be sure it really works as expected? Maybe some
> more output to get an idea what's actually going on? Or is it just a
> timing issue?

Sure, here's some added printks (and fixed posi).  You should be able to
run 'dmesg | grep pci_vc_needs_save' after boot and see device
0000:03:00.0.  Hopefully you won't see the pci_save_vc_state() printk as
you assign the device.

diff --git a/drivers/pci/vc.c b/drivers/pci/vc.c
index 7e1304d..300e126 100644
--- a/drivers/pci/vc.c
+++ b/drivers/pci/vc.c
@@ -339,6 +339,26 @@ static int pci_vc_do_save_buffer(struct pci_dev *dev, int pos,
 	return buf ? 0 : len;
 }
 
+/**
+ * pci_vc_needs_save - Determine whether a VC capability needs to be saved
+ * @dev: device
+ * @id: VC capability ID (VC/VC9/MFVC)
+ *
+ * In configurations where we have a VC or MFVC capability, but the upstream
+ * device does not, we assume that VC save (and therefore restore) is not
+ * necessary.  The intention is to only do VC save/restore in configuration
+ * where it's necessary and hopefully avoid reset issues.
+ */
+static bool pci_vc_needs_save(struct pci_dev *dev, u16 id)
+{
+	if (id == PCI_EXT_CAP_ID_VC9 || pci_is_root_bus(dev->bus) ||
+	    pci_find_ext_capability(dev->bus->self, PCI_EXT_CAP_ID_VC))
+		return true;
+
+	printk("%s(%s, %x) returning false\n", __func__, pci_name(dev), id);
+	return false;
+}
+
 static struct {
 	u16 id;
 	const char *name;
@@ -362,7 +382,7 @@ int pci_save_vc_state(struct pci_dev *dev)
 		struct pci_cap_saved_state *save_state;
 
 		pos = pci_find_ext_capability(dev, vc_caps[i].id);
-		if (!pos)
+		if (!pos || !pci_vc_needs_save(dev, vc_caps[i].id))
 			continue;
 
 		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
@@ -372,6 +392,7 @@ int pci_save_vc_state(struct pci_dev *dev)
 			return -ENOMEM;
 		}
 
+	I	printk("%s doing %s save on %s\n", __func__, vc_caps[i].name, pci_name(dev));
 		ret = pci_vc_do_save_buffer(dev, pos, save_state, true);
 		if (ret) {
 			dev_err(&dev->dev, "%s save unsuccessful %s\n",
@@ -403,6 +424,7 @@ void pci_restore_vc_state(struct pci_dev *dev)
 		if (!save_state || !pos)
 			continue;
 
+	I	printk("%s doing %s restore on %s\n", __func__, vc_caps[i].name, pci_name(dev));
 		pci_vc_do_save_buffer(dev, pos, save_state, false);
 	}
 }
@@ -422,7 +444,7 @@ void pci_allocate_vc_save_buffers(struct pci_dev *dev)
 	for (i = 0; i < ARRAY_SIZE(vc_caps); i++) {
 		int len, pos = pci_find_ext_capability(dev, vc_caps[i].id);
 
-		if (!pos)
+		if (!pos || !pci_vc_needs_save(dev, vc_caps[i].id))
 			continue;
 
 		len = pci_vc_do_save_buffer(dev, pos, NULL, false);