Date:   Fri, 21 May 2021 05:26:50 +0000
From:   Anchal Agarwal <anchalag@...zon.com>
To:     <boris.ostrovsky@...cle.com>
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Subject: Re: [PATCH v3 01/11] xen/manage: keep track of the on-going suspend mode

On Thu, Oct 01, 2020 at 08:43:58AM -0400, boris.ostrovsky@...cle.com wrote:
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> 
> 
> 
> >>>>>>> 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.
> 
Hi Boris,
Apologies for picking this up after last year. 
I did some dive deep on the above statement and that is indeed the case that's happening. 
I did some debugging around KASLR and hibernation using reboot mode.
I observed in my debug prints that whenever vcpu_info* address for secondary vcpu assigned 
in xen_vcpu_setup at boot is different than what is in the image, resume gets stuck for that vcpu
in bringup_cpu(). That means we have different addresses for &per_cpu(xen_vcpu_info, cpu) at boot and after
control jumps into the image. 

I failed to get any prints after it got stuck in bringup_cpu() and
I do not have an option to send a sysrq signal to the guest or rather get a kdump.
This change is not observed in every hibernate-resume cycle. I am not sure if this is a bug or an 
expected behavior. 
Also, I am contemplating the idea that it may be a bug in xen code getting triggered only when
KASLR is enabled but I do not have substantial data to prove that.
Is this a coincidence that this always happens for 1st vcpu?
Moreover, since hypervisor is not aware that guest is hibernated and it looks like a regular shutdown to dom0 during reboot mode,
will re-registering vcpu_info for secondary vcpu's even plausible? I could definitely use some advice to debug this further.

 
Some printk's from my debugging:

At Boot:

xen_vcpu_setup: xen_have_vcpu_info_placement=1 cpu=1, vcpup=0xffff9e548fa560e0, info.mfn=3996246 info.offset=224,

Image Loads:
It ends up in the condition:
 xen_vcpu_setup()
 {
 ...
 if (xen_hvm_domain()) {
        if (per_cpu(xen_vcpu, cpu) == &per_cpu(xen_vcpu_info, cpu))
                return 0; 
 }
 ...
 }

xen_vcpu_setup: checking mfn on resume cpu=1, info.mfn=3934806 info.offset=224, &per_cpu(xen_vcpu_info, cpu)=0xffff9d7240a560e0

This is tested on c4.2xlarge [8vcpu 15GB mem] instance with 5.10 kernel running
in the guest.

Thanks,
Anchal.
> 
> -boris
> 
> 

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