Message-Id: <C77735B9-0801-4136-B7E5-AFF02290F54D@gmail.com>
Date:   Fri, 17 Jan 2020 13:57:43 -0500
From:   Rich Persaud <persaur@...il.com>
To:     Pasi Kärkkäinen <pasik@....fi>
Cc:     "Spassov, Stanislav" <stanspas@...zon.de>,
        "jgross@...e.com" <jgross@...e.com>,
        "sstabellini@...nel.org" <sstabellini@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baijiaju1990@...il.com" <baijiaju1990@...il.com>,
        "jbeulich@...e.com" <jbeulich@...e.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "roger.pau@...rix.com" <roger.pau@...rix.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Chao Gao <chao.gao@...el.com>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>,
        Jason Andryuk <jandryuk@...il.com>
Subject: Re: [Xen-devel] [PATCH] xen: xen-pciback: Reset MSI-X state when exposing a device

On Sep 26, 2019, at 06:17, Pasi Kärkkäinen <pasik@....fi> 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@...el.com> wrote:
>>>>>> On Wed, Dec 12, 2018 at 08:21:39AM -0700, Jan Beulich wrote:
>>>>>>>>>> On 12.12.18 at 16:18, <chao.gao@...el.com> wrote:
>>>>>>>> On Wed, Dec 12, 2018 at 01:51:01AM -0700, Jan Beulich wrote:
>>>>>>>>>>>> On 12.12.18 at 08:06, <chao.gao@...el.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? 

What further work is needed for these patches?  Are they only needed for Intel i210 NIC PCI passthrough, or are other devices affected?

Rich

Powered by blists - more mailing lists