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

From: Chao Gao
Date: Wed Dec 12 2018 - 02:03:22 EST

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().