Re: [Xen-devel] [PATCH] xen: xen-pciback: Reset MSI-X state when exposing a device

["Spassov, Stanislav" <stanspas@amazon.de>]

Hello Pasi,

Unfortunately, I am not able to continue the work on the Xen patches in
the foreseeable future.

For what it's worth: the xen-pciback workaround from this thread solves
my current issue as confirmed by internal testing.

-- Stanislav

(apologies for ugly footer injected below by company SMTP server
due to local laws)

On Thu, 2019-09-26 at 13:13 +0300, Pasi Kärkkäinen wrote:
> Hello Stanislav,
> 
> On Fri, Sep 13, 2019 at 11:28:20PM +0800, Chao Gao wrote:
> > On Fri, Sep 13, 2019 at 10:02:24AM +0000, Spassov, Stanislav wrote:
> > > On Thu, Dec 13, 2018 at 07:54, Chao Gao wrote:
> > > > On Thu, Dec 13, 2018 at 12:54:52AM -0700, Jan Beulich wrote:
> > > > > > > > On 13.12.18 at 04:46, <chao.gao@intel.com> wrote:
> > > > > > 
> > > > > > On Wed, Dec 12, 2018 at 08:21:39AM -0700, Jan Beulich
> > > > > > wrote:
> > > > > > > > > > On 12.12.18 at 16:18, <chao.gao@intel.com> wrote:
> > > > > > > > 
> > > > > > > > On Wed, Dec 12, 2018 at 01:51:01AM -0700, Jan Beulich
> > > > > > > > wrote:
> > > > > > > > > > > > On 12.12.18 at 08:06, <chao.gao@intel.com>
> > > > > > > > > > > > wrote:
> > > > > > > > > > 
> > > > > > > > > > On Wed, Dec 05, 2018 at 09:01:33AM -0500, Boris
> > > > > > > > > > Ostrovsky wrote:
> > > > > > > > > > > On 12/5/18 4:32 AM, Roger Pau Monné wrote:
> > > > > > > > > > > > On Wed, Dec 05, 2018 at 10:19:17AM +0800, Chao
> > > > > > > > > > > > Gao wrote:
> > > > > > > > > > > > > I find some pass-thru devices don't work any
> > > > > > > > > > > > > more across guest reboot.
> > > > > > > > > > > > > Assigning it to another guest also meets the
> > > > > > > > > > > > > same issue. And the only
> > > > > > > > > > > > > way to make it work again is un-binding and
> > > > > > > > > > > > > binding it to pciback.
> > > > > > > > > > > > > Someone reported this issue one year ago [1].
> > > > > > > > > > > > > More detail also can be
> > > > > > > > > > > > > found in [2].
> > > > > > > > > > > > > 
> > > > > > > > > > > > > The root-cause is Xen's internal MSI-X state
> > > > > > > > > > > > > isn't reset properly
> > > > > > > > > > > > > during reboot or re-assignment. In the above
> > > > > > > > > > > > > case, Xen set maskall bit
> > > > > > > > > > > > > to mask all MSI interrupts after it detected
> > > > > > > > > > > > > a potential security
> > > > > > > > > > > > > issue. Even after device reset, Xen didn't
> > > > > > > > > > > > > reset its internal maskall
> > > > > > > > > > > > > bit. As a result, maskall bit would be set
> > > > > > > > > > > > > again in next write to
> > > > > > > > > > > > > MSI-X message control register.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Given that PHYSDEVOPS_prepare_msix() also
> > > > > > > > > > > > > triggers Xen resetting MSI-X
> > > > > > > > > > > > > internal state of a device, we employ it to
> > > > > > > > > > > > > fix this issue rather than
> > > > > > > > > > > > > introducing another dedicated sub-hypercall.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Note that PHYSDEVOPS_release_msix() will fail
> > > > > > > > > > > > > if the mapping between
> > > > > > > > > > > > > the device's msix and pirq has been created.
> > > > > > > > > > > > > This limitation prevents
> > > > > > > > > > > > > us calling this function when detaching a
> > > > > > > > > > > > > device from a guest during
> > > > > > > > > > > > > guest shutdown. Thus it is called right
> > > > > > > > > > > > > before calling
> > > > > > > > > > > > > PHYSDEVOPS_prepare_msix().
> > > > > > > > > > > > 
> > > > > > > > > > > > s/PHYSDEVOPS/PHYSDEVOP/ (no final S). And then
> > > > > > > > > > > > I would also drop the
> > > > > > > > > > > > () at the end of the hypercall name since it's
> > > > > > > > > > > > not a function.
> > > > > > > > > > > > 
> > > > > > > > > > > > I'm also wondering why the release can't be
> > > > > > > > > > > > done when the device is
> > > > > > > > > > > > detached from the guest (or the guest has been
> > > > > > > > > > > > shut down). This makes
> > > > > > > > > > > > me worry about the raciness of the
> > > > > > > > > > > > attach/detach procedure: if there's
> > > > > > > > > > > > a state where pciback assumes the device has
> > > > > > > > > > > > been detached from the
> > > > > > > > > > > > guest, but there are still pirqs bound, an
> > > > > > > > > > > > attempt to attach to
> > > > > > > > > > > > another guest in such state will fail.
> > > > > > > > > > > 
> > > > > > > > > > > I wonder whether this additional reset
> > > > > > > > > > > functionality could be done out
> > > > > > > > > > > of xen_pcibk_xenbus_remove(). We first do a (best
> > > > > > > > > > > effort) device reset
> > > > > > > > > > > and then do the extra things that are not
> > > > > > > > > > > properly done there.
> > > > > > > > > > 
> > > > > > > > > > No. It cannot be done in xen_pcibk_xenbus_remove()
> > > > > > > > > > without modifying
> > > > > > > > > > the handler of PHYSDEVOP_release_msix. To do a
> > > > > > > > > > successful Xen internal
> > > > > > > > > > MSI-X state reset, PHYSDEVOP_{release,
> > > > > > > > > > prepare}_msix should be finished
> > > > > > > > > > without error. But ATM, xen expects that no msi is
> > > > > > > > > > bound to pirq when
> > > > > > > > > > doing PHYSDEVOP_release_msix. Otherwise it fails
> > > > > > > > > > with error code -EBUSY.
> > > > > > > > > > However, the expectation isn't guaranteed in
> > > > > > > > > > xen_pcibk_xenbus_remove().
> > > > > > > > > > In some cases, if qemu fails to unmap MSIs, MSIs
> > > > > > > > > > are unmapped by Xen
> > > > > > > > > > at last minute, which happens after device reset
> > > > > > > > > > in 
> > > > > > > > > > xen_pcibk_xenbus_remove().
> > > > > > > > > 
> > > > > > > > > But that may need taking care of: I don't think it is
> > > > > > > > > a good idea to have
> > > > > > > > > anything left from the prior owning domain when the
> > > > > > > > > device gets reset.
> > > > > > > > > I.e. left over IRQ bindings should perhaps be
> > > > > > > > > forcibly cleared before
> > > > > > > > > invoking the reset;
> > > > > > > > 
> > > > > > > > Agree. How about pciback to track the established IRQ
> > > > > > > > bindings? Then
> > > > > > > > pciback can clear irq binding before invoking the
> > > > > > > > reset.
> > > > > > > 
> > > > > > > How would pciback even know of those mappings, when it's
> > > > > > > qemu
> > > > > > > who establishes (and manages) them?
> > > > > > 
> > > > > > I meant to expose some interfaces from pciback. And pciback
> > > > > > serves
> > > > > > as the proxy of IRQ (un)binding APIs.
> > > > > 
> > > > > If at all possible we should avoid having to change more
> > > > > parties (qemu,
> > > > > libxc, kernel, hypervisor) than really necessary. Remember
> > > > > that such
> > > > > a bug fix may want backporting, and making sure affected
> > > > > people have
> > > > > all relevant components updated is increasingly difficult
> > > > > with their
> > > > > number growing.
> > > > > 
> > > > > > > > > in fact I'd expect this to happen in the course of
> > > > > > > > > domain destruction, and I'd expect the device reset
> > > > > > > > > to come after the
> > > > > > > > > domain was cleaned up. Perhaps simply an ordering
> > > > > > > > > issue in the tool
> > > > > > > > > stack?
> > > > > > > > 
> > > > > > > > I don't think reversing the sequences of device reset
> > > > > > > > and domain
> > > > > > > > destruction would be simple. Furthermore, during device
> > > > > > > > hot-unplug,
> > > > > > > > device reset is done when the owner is alive. So if we
> > > > > > > > use domain
> > > > > > > > destruction to enforce all irq binding cleared, in
> > > > > > > > theory, it won't be
> > > > > > > > applicable to hot-unplug case (if qemu's hot-unplug
> > > > > > > > logic is
> > > > > > > > compromised).
> > > > > > > 
> > > > > > > Even in the hot-unplug case the tool stack could issue
> > > > > > > unbind
> > > > > > > requests, behind the back of the possibly compromised
> > > > > > > qemu,
> > > > > > > once neither the guest nor qemu have access to the device
> > > > > > > anymore.
> > > > > > 
> > > > > > But currently, tool stack doesn't know the remaining IRQ
> > > > > > bindings.
> > > > > > If tool stack can maintaine IRQ binding information of a
> > > > > > pass-thru
> > > > > > device (stored in Xenstore?), we can come up with a clean
> > > > > > solution
> > > > > > without modifying linux kernel and Xen.
> > > > > 
> > > > > If there's no way for the tool stack to either find out the
> > > > > bindings
> > > > > or "blindly" issue unbind requests (accepting them to fail),
> > > > > then a
> > > > > "wildcard" unbind operation may want adding. Or, perhaps even
> > > > > better, XEN_DOMCTL_deassign_device could unbind anything left
> > > > > in place for the specified device.
> > > > 
> > > > Good idea. I will take this advice.
> > > > 
> > > > Thanks
> > > > Chao
> > > 
> > > I am having the same issue, and cannot find a fix in either xen-
> > > pciback or the Xen codebase.
> > > Was a solution ever pushed as a result of this thread?
> > > 
> > 
> > I submitted patches [1] to Xen community. But I didn't get it
> > merged.
> > We made a change in device driver to disable MSI-X during guest OS
> > shutdown to mitigate the issue. But when guest or qemu was crashed,
> > we
> > encountered this issue again. I have no plan to get back to these
> > patches. But if you want to fix the issue completely along what the
> > patches below did, please go ahead.
> > 
> > [1]: 
> > https://lists.xenproject.org/archives/html/xen-devel/2019-01/msg01227.html
> > 
> > Thanks
> > Chao
> > 
> 
> Stanislav: Are you able to continue the work with these patches, to
> get them merged? 
> 
> 
> Thanks,
> 
> -- Pasi
> 



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