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

From: boris.ostrovsky@oracle.com
To: Anchal Agarwal <anchalag@amazon.com>
Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
	hpa@zytor.com, x86@kernel.org, jgross@suse.com,
	linux-pm@vger.kernel.org, linux-mm@kvack.org, kamatam@amazon.com,
	sstabellini@kernel.org, konrad.wilk@oracle.com,
	roger.pau@citrix.com, axboe@kernel.dk, davem@davemloft.net,
	rjw@rjwysocki.net, len.brown@intel.com, pavel@ucw.cz,
	peterz@infradead.org, eduval@amazon.com, sblbir@amazon.com,
	xen-devel@lists.xenproject.org, vkuznets@redhat.com,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	dwmw@amazon.co.uk, benh@kernel.crashing.org
Subject: Re: [PATCH v3 01/11] xen/manage: keep track of the on-going suspend mode
Date: Mon, 28 Sep 2020 14:49:56 -0400	[thread overview]
Message-ID: <cc738014-6a79-a5ae-cb2a-a02ff15b4582@oracle.com> (raw)
In-Reply-To: <20200925222826.GA11755@dev-dsk-anchalag-2a-9c2d1d96.us-west-2.amazon.com>

On 9/25/20 6:28 PM, Anchal Agarwal wrote:
> On Fri, Sep 25, 2020 at 04:02:58PM -0400, boris.ostrovsky@oracle.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@oracle.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