Date:   Thu, 15 Dec 2016 00:25:13 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Cao jin <caoj.fnst@...fujitsu.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, izumi.taku@...fujitsu.com
Subject: Re: [PATCH] vfio/pci: Support error recovery

On Wed, Dec 14, 2016 at 03:16:37PM -0700, Alex Williamson wrote:
> On Wed, 14 Dec 2016 18:24:23 +0800
> Cao jin <caoj.fnst@...fujitsu.com> wrote:
> 
> > Sorry for late.
> > after reading all your comments, I think I will try the solution 1.
> > 
> > On 12/13/2016 03:12 AM, Alex Williamson wrote:
> > > On Mon, 12 Dec 2016 21:49:01 +0800
> > > Cao jin <caoj.fnst@...fujitsu.com> wrote:
> > >   
> > >> Hi,
> > >> I have 2 solutions(high level design) came to me, please see if they are
> > >> acceptable, or which one is acceptable. Also have some questions.
> > >>
> > >> 1. block guest access during host recovery
> > >>
> > >>    add new field error_recovering in struct vfio_pci_device to
> > >>    indicate host recovery status. aer driver in host will still do
> > >>    reset link
> > >>
> > >>    - set error_recovering in vfio-pci driver's error_detected, used to
> > >>      block all kinds of user access(config space, mmio)
> > >>    - in order to solve concurrent issue of device resetting & user
> > >>      access, check device state[*] in vfio-pci driver's resume, see if
> > >>      device reset is done, if it is, then clear"error_recovering", or
> > >>      else new a timer, check device state periodically until device
> > >>      reset is done. (what if device reset don't end for a long time?)
> > >>    - In qemu, translate guest link reset to host link reset.
> > >>      A question here: we already have link reset in host, is a second
> > >>      link reset necessary? why?
> > >>  
> > >>    [*] how to check device state: reading certain config space
> > >>        register, check return value is valid or not(All F's)  
> > > 
> > > Isn't this exactly the path we were on previously?  
> > 
> > Yes, it is basically the previous path, plus the optimization.
> > 
> > > There might be an
> > > optimization that we could skip back-to-back resets, but how can you
> > > necessarily infer that the resets are for the same thing? If the user
> > > accesses the device between resets, can you still guarantee the guest
> > > directed reset is unnecessary?  If time passes between resets, do you
> > > know they're for the same event?  How much time can pass between the
> > > host and guest reset to know they're for the same event?  In the
> > > process of error handling, which is more important, speed or
> > > correctness?
> > >    
> > 
> > I think vfio driver itself won't know what each reset comes for, and I
> > don't quite understand why should vfio care this question, is this a new
> > question in the design?
> 
> You're suggesting an optimization to eliminate one of the resets,
> and as we've discussed, I don't see removing the host induced reset
> as a viable option.  That means you want to eliminate the guest
> directed reset.  There are potentially three levels to do that, the
> vfio-pci driver in the host kernel, somewhere in QEMU, or eliminate it
> within the guest.  My comments were directed to the first option, the
> host kernel level cannot correlate user directed resets as duplicates
> of host directed resets.  
>  
> > But I think it make sense that the user access during 2 resets maybe a
> > trouble for guest recovery, misbehaved user could be out of our
> > imagination.  Correctness is more important.
> > 
> > If I understand you right, let me make a summary: host recovery just
> > does link reset, which is incomplete, so we'd better do a complete guest
> > recovery for correctness.
> 
> We don't know whether the host link reset is incomplete, but we can't do
> a link reset transparently to the device, the device is no longer in the
> same state after the reset.  The device specific driver, which exists
> in userspace needs to be involved in device recovery.  Therefore
> regardless of how QEMU handles the error, the driver within the guest
> needs to be notified and perform recovery.  Since the device is PCI and
> we're on x86 and nobody wants to introduce paravirtual error recovery,
> we must use AER.  Part of AER recovery includes the possibility of
> performing a link reset.  So it seems this eliminates avoiding the link
> reset within the guest.
> 
> That leaves QEMU.  Here we need to decide whether a guest triggered
> link reset induces a host link reset.  The current working theory is
> that yes, this must be the case.  If there is ever a case where a
> driver within the guest could trigger a link reset for the purposes
> of error recovery when the host has not, I think this must be the
> case.  Therefore, at least some guest induced link resets must become
> host link resets.  Currently we assume all guest induced link resets
> become host link resets.  Minimally to avoid that, QEMU would need to
> know (not assume) whether the host performed a link reset.  Even with
> that, QEMU would need to be able to correlate that a link reset from
> the guest is a duplicate of a link reset that was already performed by
> the host.  That implies that QEMU needs to deduce the intention of
> the guest.  That seems like a complicated task for a patch series that
> is already complicated enough, especially for a feature of questionable
> value given the configuration restrictions (imo).
> 
> I would much rather focus on getting it right and making it as simple
> as we can, even if that means links get reset one too many times on
> error.
> 
> > >> 2. skip link reset in aer driver of host kernel, for vfio-pci.
> > >>    Let user decide how to do serious recovery
> > >>
> > >>    add new field "user_driver" in struct pci_dev, used to skip link
> > >>    reset for vfio-pci; add new field "link_reset" in struct
> > >>    vfio_pci_device to indicate link has been reset or not during
> > >>    recovery
> > >>
> > >>    - set user_driver in vfio_pci_probe(), to skip link reset for
> > >>      vfio-pci in host.
> > >>    - (use a flag)block user access(config, mmio) during host recovery
> > >>      (not sure if this step is necessary)
> > >>    - In qemu, translate guest link reset to host link reset.
> > >>    - In vfio-pci driver, set link_reset after VFIO_DEVICE_PCI_HOT_RESET
> > >>      is executed
> > >>    - In vfio-pci driver's resume, new a timer, check "link_reset" field
> > >>      periodically, if it is set in reasonable time, then clear it and
> > >>      delete timer, or else, vfio-pci driver will does the link reset!  
> > > 
> > > What happens in the case of a multifunction device where each function
> > > is part of a separate IOMMU group and one function is hot-removed from
> > > the user? We can't do a link reset on that function since the other
> > > function is still in use.  We have no choice but release a device in an
> > > unknown state back to the host.  
> > 
> > hot-remove from user, do you mean, for example, all functions assigned
> > to VM, then suddenly a person does something like following
> > 
> > $ echo 0000:06:00.0 > /sys/bus/pci/drivers/vfio-pci/unbind
> > 
> > $ echo 0000:06:00.0 > /sys/bus/pci/drivers/igb/bind
> > 
> > to return device to host driver, or don't bind it to host driver, let it
> > in driver-less state???
> 
> Yes, the host kernel has no visiblity to how a user is making use of
> devices.  To support AER we require a similar topology between host and
> guest such that a guest link reset translates to a host reset.  That
> requirement is imposed by userspace, ie. QEMU.  The host kernel cannot
> presume that this is the case.

So enforce this to enable recovery functionality. Why can't you?

>  Therefore we could have a
> multi-function device where each function is assigned to the same or
> different users in any configuration.  If a fault occurs and we defer
> to the user to perform the link reset, we have absolutely no guarantee
> that it will ever occur.  If the functions are assigned to different
> users, then each user individually doesn't have the capability to
> perform a link reset.  If the devices happen to be assigned to a single
> user when the error occurs, we cannot assume the user has an AER
> compatible configuration, the devices could be exposed as separate
> single function devices, any one of which might be individually removed
> from the user and made use of by the host, such as your sysfs example
> above.  The host cannot perform a link reset in this case either
> as the sibling devices are still in use by the guest.  Thanks,
> 
> Alex

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