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

From: Chao Gao
Date: Wed Dec 12 2018 - 22:43:21 EST

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.

>>>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
>> 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

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.