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

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

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?

Best,
Stanislav



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