linux-kernel - Re: Problems with Zen under Xen and recent Linux kernel improvements

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Date:   Tue, 31 Jul 2018 13:58:27 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Adam Novak <interfect@...il.com>, linux-kernel@...r.kernel.org
Cc:     boris.ostrovsky@...cle.com, x86@...nel.org
Subject: Re: Problems with Zen under Xen and recent Linux kernel improvements

On 31/07/18 03:14, Adam Novak wrote:
> Hello,
> 
> I was advised to take this here, and to Boris Ostrovsky and Juergen
> Gross, by Thomas Gleixner.
> 
> I am having some trouble with the new speculation control code that
> has been added to the Linux kernel, for AMD Zen CPUs. I am running an
> AMD Ryzen 7 1700, and I am running Linux as a Xen dom0 (which is part
> of the problem; the code seems to work fine running outside of Xen).
> 
> I started having trouble on Ubuntu's commit
> 3f6a3b035f91a22c0d3bd27630bf61eac9c8cf6c, "x86/speculation: Handle HT
> correctly on AMD", which appears to be cherry-picked from
> 1f50ddb4f4189243c05926b842dc1a0332195f31. Since that commit, my system
> hangs during the boot process; it starts starting stuff up and trying
> to mount things and printing "[OK]" messages, but then fairly early in
> the boot process the kernel complains that it is "unable to handle
> kernel NULL pointer deference at 000...0008"
> 
> On my Ubuntu bug:
> 
> https://bugs.launchpad.net/bugs/1777338
> 
> I have a "Screenshot of the null pointer dereference message". It is
> running into trouble during a spin lock in the new
> speculative_store_bypass_update().
> 
> Has anyone else seen this behavior on these CPUs under Xen (I am using 4.9)?

You want at least 4.9.112, especially due to the missing patches
"x86/xen: Add call of speculative_store_bypass_ht_init() to PV paths",
"x86/cpu: Re-apply forced caps every time CPU caps are re-read"

Juergen