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

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Date:   Mon, 28 Sep 2020 14:49:56 -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


On 9/25/20 6:28 PM, Anchal Agarwal wrote:
> On Fri, Sep 25, 2020 at 04:02:58PM -0400, boris.ostrovsky@...cle.com wrote:
>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>
>>
>>
>> On 9/25/20 3:04 PM, Anchal Agarwal wrote:
>>> On Tue, Sep 22, 2020 at 11:17:36PM +0000, Anchal Agarwal wrote:
>>>> On Tue, Sep 22, 2020 at 12:18:05PM -0400, boris.ostrovsky@...cle.com wrote:
>>>>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>>>>
>>>>>
>>>>>
>>>>> On 9/21/20 5:54 PM, Anchal Agarwal wrote:

>>>>> 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.


3% failure for this feature seems to be a manageable number from the reproducability perspective --- you should be able to script this and each iteration should take way under a minute, no?


> 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.


> The issue was only
> reproducible 3% of the time out of 3000 runs hence its hard to just reproduce this.
>
> Moreover, I also wanted to get an insight on if hibernation works correctly with KASLR
> generally and its only Xen causing the issue?


With KASLR being on by default I'd be surprised if it didn't.


-boris