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

[Chao Gao]

On Thu, Dec 13, 2018 at 12:54:52AM -0700, Jan Beulich wrote:
>>>> On 13.12.18 at 04:46, <chao.gao@xxxxxxxxx> wrote:
>> On Wed, Dec 12, 2018 at 08:21:39AM -0700, Jan Beulich wrote:
>>>>>> On 12.12.18 at 16:18, <chao.gao@xxxxxxxxx> wrote:
>>>> On Wed, Dec 12, 2018 at 01:51:01AM -0700, Jan Beulich wrote:
>>>>>>>> On 12.12.18 at 08:06, <chao.gao@xxxxxxxxx> 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