Date:   Tue, 28 Mar 2017 10:12:33 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Cao jin <caoj.fnst@...fujitsu.com>
Cc:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <qemu-devel@...gnu.org>, <izumi.taku@...fujitsu.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v6] vfio error recovery: kernel support

On Tue, 28 Mar 2017 21:47:00 +0800
Cao jin <caoj.fnst@...fujitsu.com> wrote:

> On 03/25/2017 06:12 AM, Alex Williamson wrote:
> > On Thu, 23 Mar 2017 17:07:31 +0800
> > Cao jin <caoj.fnst@...fujitsu.com> wrote:
> > 
> > A more appropriate patch subject would be:
> > 
> > vfio-pci: Report correctable errors and slot reset events to user
> >  
> 
> Correctable? It is confusing to me. Correctable error has its clear
> definition in PCIe spec, shouldn't it be "non-fatal"?

My mistake, non-fatal.
 
> >> From: "Michael S. Tsirkin" <mst@...hat.com>  
> > 
> > This hardly seems accurate anymore.  You could say Suggested-by and let
> > Michael add a sign-off, but it's changed since he sent it.
> >   
> >>
> >> 0. What happens now (PCIE AER only)
> >>    Fatal errors cause a link reset. Non fatal errors don't.
> >>    All errors stop the QEMU guest eventually, but not immediately,
> >>    because it's detected and reported asynchronously.
> >>    Interrupts are forwarded as usual.
> >>    Correctable errors are not reported to user at all.
> >>
> >>    Note:
> >>    PPC EEH is different, but this approach won't affect EEH. EEH treat
> >>    all errors as fatal ones in AER, so they will still be signalled to user
> >>    via the legacy eventfd.  Besides, all devices/functions in a PE belongs
> >>    to the same IOMMU group, so the slot_reset handler in this approach
> >>    won't affect EEH either.
> >>
> >> 1. Correctable errors
> >>    Hardware can correct these errors without software intervention,
> >>    clear the error status is enough, this is what already done now.
> >>    No need to recover it, nothing changed, leave it as it is.
> >>
> >> 2. Fatal errors
> >>    They will induce a link reset. This is troublesome when user is
> >>    a QEMU guest. This approach doesn't touch the existing mechanism.
> >>
> >> 3. Non-fatal errors
> >>    Before this patch, they are signalled to user the same way as fatal ones.
> >>    With this patch, a new eventfd is introduced only for non-fatal error
> >>    notification. By splitting non-fatal ones out, it will benefit AER
> >>    recovery of a QEMU guest user.
> >>
> >>    To maintain backwards compatibility with userspace, non-fatal errors
> >>    will continue to trigger via the existing error interrupt index if a
> >>    non-fatal signaling mechanism has not been registered.
> >>
> >>    Note:
> >>    In case of PCI Express errors, kernel might request a slot reset
> >>    affecting our device (from our point of view this is a passive device
> >>    reset as opposed to an active one requested by vfio itself).
> >>    This might currently happen if a slot reset is requested by a driver
> >>    (other than vfio) bound to another device function in the same slot.
> >>    This will cause our device to lose its state so report this event to
> >>    userspace.  
> > 
> > I tried to convey this in my last comments, I don't think this is an
> > appropriate commit log.  Lead with what is the problem you're trying to
> > fix and why, what is the benefit to the user, and how is the change
> > accomplished.  If you want to provide a State of Error Handling in
> > VFIO, append it after the main points of the commit log.  
> 
> ok.
> 
> > 
> > I also asked in my previous comments to provide examples of errors that
> > might trigger correctable errors to the user, this comment seems to
> > have been missed.  In my experience, AERs generated during device
> > assignment are generally hardware faults or induced by bad guest
> > drivers.  These are cases where a single fatal error is an appropriate
> > and sufficient response.  We've scaled back this support to the point
> > where we're only improving the situation of correctable errors and I'm
> > not convinced this is worthwhile and we're not simply checking a box on
> > an ill-conceived marketing requirements document.  
> 
> Sorry. I noticed that question: "what actual errors do we expect
> userspace to see as non-fatal errors?", but I am confused about it.
> Correctable, non-fatal, fatal errors are clearly defined in PCIe spec,
> and Uncorrectable Error Severity Register will tell which is fatal, and
> which is non-fatal, this register is configurable, they are device
> specific as I guess. AER core driver distinguish them by
> pci_channel_io_normal/pci_channel_io_frozen,  So I don't understand your
> question. Or
> 
> Or, Do you mean we could list the default non-fatal error of
> Uncorrectable Error Severity Register which is provided by PCIe spec?

I'm trying to ask why is this patch series useful.  It's clearly
possible for us to signal non-fatal errors for a device to a guest, but
why is it necessarily a good idea to do so?  What additional RAS
feature is gained by this?  Can we give a single example of a real
world scenario where a guest has been shutdown due to a non-fatal error
that the guest driver would have handled?

> > I had also commented asking how the hypervisor is expected to know
> > whether the guest supports AER.  With the existing support of a single
> > fatal error, the hypervisor halts the VM regardless of the error
> > severity or guest support.  Now we have the opportunity that the
> > hypervisor can forward a correctable error to the guest... and hope the
> > right thing occurs?  I never saw any response to this comment.
> >   
> 
> I noticed this question, you said: "That doesn't imply a problem with
> this approach, the user (hypervisor) would be at fault for any
> difference in handling in that case.". Maybe I understand you wrong.
> 
> From my limit understanding, QEMU doesn't has a way to know whether a
> guest has AER support, AER support need several kbuild configuration, I
> don't know how qemu is expected to know these.


Isn't that a problem?  See my reply to QEMU patch 3/3.
 
> >>
> >> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> >> Signed-off-by: Cao jin <caoj.fnst@...fujitsu.com>
> >> ---
> >> v6 changelog:
> >> Address all the comments from MST.
> >>
> >>  drivers/vfio/pci/vfio_pci.c         | 49 +++++++++++++++++++++++++++++++++++--
> >>  drivers/vfio/pci/vfio_pci_intrs.c   | 38 ++++++++++++++++++++++++++++
> >>  drivers/vfio/pci/vfio_pci_private.h |  2 ++
> >>  include/uapi/linux/vfio.h           |  2 ++
> >>  4 files changed, 89 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> >> index 324c52e..71f9a8a 100644
> >> --- a/drivers/vfio/pci/vfio_pci.c
> >> +++ b/drivers/vfio/pci/vfio_pci.c
> >> @@ -441,7 +441,9 @@ static int vfio_pci_get_irq_count(struct vfio_pci_device *vdev, int irq_type)
> >>  
> >>  			return (flags & PCI_MSIX_FLAGS_QSIZE) + 1;
> >>  		}
> >> -	} else if (irq_type == VFIO_PCI_ERR_IRQ_INDEX) {
> >> +	} else if (irq_type == VFIO_PCI_ERR_IRQ_INDEX ||
> >> +		   irq_type == VFIO_PCI_NON_FATAL_ERR_IRQ_INDEX ||
> >> +		   irq_type == VFIO_PCI_PASSIVE_RESET_IRQ_INDEX) {  
> > 
> > Should we add a typdef to alias VFIO_PCI_ERR_IRQ_INDEX to
> > VFIO_PCI_FATAL_ERR_IRQ?
> >   
> >>  		if (pci_is_pcie(vdev->pdev))
> >>  			return 1;
> >>  	} else if (irq_type == VFIO_PCI_REQ_IRQ_INDEX) {
> >> @@ -796,6 +798,8 @@ static long vfio_pci_ioctl(void *device_data,
> >>  		case VFIO_PCI_REQ_IRQ_INDEX:
> >>  			break;
> >>  		case VFIO_PCI_ERR_IRQ_INDEX:
> >> +		case VFIO_PCI_NON_FATAL_ERR_IRQ_INDEX:
> >> +		case VFIO_PCI_PASSIVE_RESET_IRQ_INDEX:
> >>  			if (pci_is_pcie(vdev->pdev))
> >>  				break;
> >>  		/* pass thru to return error */
> >> @@ -1282,7 +1286,9 @@ static pci_ers_result_t vfio_pci_aer_err_detected(struct pci_dev *pdev,
> >>  
> >>  	mutex_lock(&vdev->igate);
> >>  
> >> -	if (vdev->err_trigger)
> >> +	if (state == pci_channel_io_normal && vdev->non_fatal_err_trigger)
> >> +		eventfd_signal(vdev->non_fatal_err_trigger, 1);
> >> +	else if (vdev->err_trigger)
> >>  		eventfd_signal(vdev->err_trigger, 1);  
> > 
> > Should another patch rename err_trigger to fatal_err_trigger to better
> > describe its new function?
> >   
> >>  
> >>  	mutex_unlock(&vdev->igate);
> >> @@ -1292,8 +1298,47 @@ static pci_ers_result_t vfio_pci_aer_err_detected(struct pci_dev *pdev,
> >>  	return PCI_ERS_RESULT_CAN_RECOVER;
> >>  }
> >>  
> >> +/*
> >> + * In case of PCI Express errors, kernel might request a slot reset
> >> + * affecting our device (from our point of view, this is a passive device
> >> + * reset as opposed to an active one requested by vfio itself).
> >> + * This might currently happen if a slot reset is requested by a driver
> >> + * (other than vfio) bound to another device function in the same slot.
> >> + * This will cause our device to lose its state, so report this event to
> >> + * userspace.
> >> + */  
> > 
> > I really dislike "passive reset".  I expect you avoided "slot reset"
> > because we have other sources where vfio itself initiates a slot
> > reset.  Is "spurious" more appropriate?  "Collateral"?
> >   
> >> +static pci_ers_result_t vfio_pci_aer_slot_reset(struct pci_dev *pdev)
> >> +{
> >> +	struct vfio_pci_device *vdev;
> >> +	struct vfio_device *device;
> >> +	static pci_ers_result_t err = PCI_ERS_RESULT_NONE;
> >> +
> >> +	device = vfio_device_get_from_dev(&pdev->dev);
> >> +	if (!device)
> >> +		goto err_dev;
> >> +
> >> +	vdev = vfio_device_data(device);
> >> +	if (!vdev)
> >> +		goto err_data;
> >> +
> >> +	mutex_lock(&vdev->igate);
> >> +
> >> +	if (vdev->passive_reset_trigger)
> >> +		eventfd_signal(vdev->passive_reset_trigger, 1);  
> > 
> > What are the exact user requirements here, we now have:
> > 
> > A) err_trigger
> > B) non_fatal_err_trigger
> > C) passive_reset_trigger
> > 
> > Currently we only have A, which makes things very simple, we notify on
> > errors and assume the user doesn't care otherwise.
> > 
> > The expectation here seems to be that A, B, and C are all registered,
> > but what if they're not?  What if in the above function, only A & B are
> > registered, do we trigger A here?  Are B & C really intrinsic to each
> > other and we should continue to issue only A unless both B & C are
> > registered?  In that case, should we be exposing a single IRQ INDEX to
> > the user with two sub-indexes, defined as sub-index 0 is correctable
> > error, sub-index 1 is slot reset, and promote any error to A if they
> > are not both registered?
> >   
> 
> I will see how to implement these.
> 

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