linux-kernel - Re: [PATCH v3 01/11] xen/manage: keep track of the on-going suspend mode

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Date:   Thu, 1 Oct 2020 08:43:58 -0400
From:   boris.ostrovsky@...cle.com
To:     Anchal Agarwal <anchalag@...zon.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, jgross@...e.com, linux-pm@...r.kernel.org,
        linux-mm@...ck.org, kamatam@...zon.com, sstabellini@...nel.org,
        konrad.wilk@...cle.com, roger.pau@...rix.com, axboe@...nel.dk,
        davem@...emloft.net, rjw@...ysocki.net, len.brown@...el.com,
        pavel@....cz, peterz@...radead.org, eduval@...zon.com,
        sblbir@...zon.com, xen-devel@...ts.xenproject.org,
        vkuznets@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, dwmw@...zon.co.uk,
        benh@...nel.crashing.org
Subject: Re: [PATCH v3 01/11] xen/manage: keep track of the on-going suspend
 mode


>>>>>>> Also, wrt KASLR stuff, that issue is still seen sometimes but I haven't had
>>>>>>> bandwidth to dive deep into the issue and fix it.
>>>> So what's the plan there? You first mentioned this issue early this year and judged by your response it is not clear whether you will ever spend time looking at it.
>>>>
>>> I do want to fix it and did do some debugging earlier this year just haven't
>>> gotten back to it. Also, wanted to understand if the issue is a blocker to this
>>> series?
>>
>> Integrating code with known bugs is less than ideal.
>>
> So for this series to be accepted, KASLR needs to be fixed along with other
> comments of course? 


Yes, please.



>>> I had some theories when debugging around this like if the random base address picked by kaslr for the
>>> resuming kernel mismatches the suspended kernel and just jogging my memory, I didn't find that as the case.
>>> Another hunch was if physical address of registered vcpu info at boot is different from what suspended kernel
>>> has and that can cause CPU's to get stuck when coming online.
>>
>> I'd think if this were the case you'd have 100% failure rate. And we are also re-registering vcpu info on xen restore and I am not aware of any failures due to KASLR.
>>
> What I meant there wrt VCPU info was that VCPU info is not unregistered during hibernation,
> so Xen still remembers the old physical addresses for the VCPU information, created by the
> booting kernel. But since the hibernation kernel may have different physical
> addresses for VCPU info and if mismatch happens, it may cause issues with resume. 
> During hibernation, the VCPU info register hypercall is not invoked again.


I still don't think that's the cause but it's certainly worth having a look.


-boris