From mboxrd@z Thu Jan  1 00:00:00 1970
From: Pavel Fedin <p.fedin@samsung.com>
Subject: RE: BUG ALERT: ARM32 KVM does not work in 4.4-rc3
Date: Tue, 01 Dec 2015 15:09:24 +0300
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To: 'Marc Zyngier' <marc.zyngier@arm.com>, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org
Cc: 'Ard Biesheuvel' <ard.biesheuvel@linaro.org>, christoffer.dall@linaro.org
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Hello!

> -----Original Message-----
> From: kvm-owner@vger.kernel.org [mailto:kvm-owner@vger.kernel.org] On Behalf Of Pavel Fedin
> Sent: Tuesday, December 01, 2015 1:03 PM
> To: 'Marc Zyngier'; kvmarm@lists.cs.columbia.edu; kvm@vger.kernel.org
> Cc: 'Ard Biesheuvel'; christoffer.dall@linaro.org
> Subject: RE: BUG ALERT: ARM32 KVM does not work in 4.4-rc3
> 
>  Hello!
> 
> > > My project involves ARM64, but from time to time i also test ARM32
> > > KVM. I have discovered that it stopped working in 4.4-rc3. The same
> > > virtual machine works perfectly under current kvmarm/next, but gets
> > > stuck at random point under 4.4-rc3 from linux-stable. I'm not sure
> > > that i have time to investigate this quickly, but i'll post some new
> > > information as soon as i get it
> 
> [skip]
> 
> > So until you bisect it to an exact commit and configuration, I declare
> > the alert over. ;-)
> 
>  The commit in question is e6fab54423450d699a09ec2b899473a541f61971 ("ARM/arm64: KVM: test
> properly for a PTE's uncachedness").
> Reverting it fixes the problem.
>  Study in qemu shows that the CPU gets stuck at PC = 0x0C with LR = 0x10. So i quickly decided
> that it might have to do with
> caching, and my first hit was correct. The guest crashes in this state very early, sometimes
> it even cannot fully print
> "Uncompressing kernel".
>  The machine which reproduces it is custom Samsung's out-of-tree board. I'll investigate it
> further in order to determine how
> exactly the commit could harm. I know that it passed reviews and testing, and i was involved
> too. Perhaps it's board's code fault,
> however.
> 
> Cc'ed to others involved.

 The problem seems to be triggered by ARCH_SPARSEMEM_ENABLE. My top-secret machine uses it, while more widespread things like
vexpress and Exynos don't.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia